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Distributed Control System and Programmable Logic Control
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Course Aim

The aim of this training course is to build up the procedural and
declarative knowledge required to be recognized by projects engineer that
don not have past background of DCS or PLC. This will help them to
supervise projects dealing with control systems with a strong background.
In this course, the training cycle is divided in five steps that necessitate
the cooperation between the instructor and the trainees. These steps are
shown in figure below, they are summarized as follows:
1. Define the knowledge and skills required to be developed.
2. Define the elements of each knowledge or skill.
3. Formulate a verbal phrase for the learning objective of each
element.
4. Choose an adequate instructional activity to present each element.
5. Set up an indicator to measure the outcomes of the course and
modify the training skills to adapt the vocational needs.

Determine
Elements
Learning
Objectives
Define
Knowledge
& Skills
Instruction
Activity
Measure
& Correction
Determine
Elements
Learning
Objectives
Define
Knowledge
& Skills
Instruction
Activity
Measure
& Correction

Training Cycle.

Distributed Control System and Programmable Logic Control
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Knowledge and Elements

 Illustrate DCS & PLC Benefits, Usage and History.
 Overview of control system history.
 Control system benefits and usage.
 Types of control

 Develop Knowledge of DCS Components (Hardware & Software).
 Infrastructure [Communication Bus, Interfaces, Controllers,
Gateways, RTU, Others].
 Hardware and technologies.
 Software [Configuration, Graphics, Alarming, Trending, System
Management, Others].

 Extend Knowledge of DCS installation and Maintenance.
 Site Installation, Commissioning and Startup.
 Diagnostics, Spares, Tools and Power Distribution.
 Maintenance [Backup, Replacements and System Installation].

 Develop Knowledge of PLC Components.
 PLC fundamentals.
 PLC Logic.

Distributed Control System and Programmable Logic Control
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Table of Contents

Section I
Chapter 1 Introduction
Chapter 2 Regulatory Control
Section II
Chapter 3 DCS Infrastructure
Chapter 4 DCS Hardware
Chapter 5 DCS Software
Section III
Chapter 6 Installation
Chapter 7 Maintenance
Chapter 8 Power Distribution
Section IV
Chapter 9 PLC Fundamentals.
Chapter 10 Ladder Logic And SFC

Appendices
A Electrical Relay Diagram And P&ID Symbols
B Serial Communication
C Networking
D Software Engineering

Distributed Control System and Programmable Logic Control
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Distributed Control System and Programmable Logic Control
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Chapter 1
Control Systems

1.1 Automation System Structure

Although applications differ widely, there is little difference in the
overall architecture of their control systems. Why the control system of a
power plant is not sold also for automating a brewery depends largely on
small differences (e.g. explosion-proof), on regulations (e.g. Food and
Drug Administration) and also tradition, customer relationship.
The ANSI/ISA standard 95 defines terminology and good practices

1.1.1 Large Control System Hierarchy

 Administration: Production goals, planning
 Enterprise: Manages resources, workflow, coordinates activities of
different sites, quality supervision, maintenance, distribution and
planning.
Enterprise Resource
Planning
Manufacturing Execution
System
Control & Command
System
Business Planning & Logistics
Plant Production Scheduling
Operational Management, etc.
Manufacturing
Operations & Control
Dispatching Production, Detailed Product
Scheduling, Reliability Assurance,...
Level
4
Level
3
Level
2,1,0
Batch
Control
Continuous
Control
Discrete
Control

Distributed Control System and Programmable Logic Control
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 Supervision: Supervision of the site, optimization, on-line
operations. Control room, Process Data Base, logging (open loop)
 Group (Area): Control of a well-defined part of the plant. closed
loop, except for intervention of an operator)
o Coordinates individual subgroups
o Adjusting set-points and parameters
o Commands several units as a whole
 Unit (Cell): Control (regulation, monitoring and protection) of a
small part of a group (closed loop except for maintenance).
o Measure: Sampling, scaling, processing, calibration.
o Control: regulation, set-points and parameters
o Command: sequencing, protection and interlocking
 Field: Sensors & Actors, data acquisition, digitalization, data
transmission, no processing except measurement correction and
built-in protection.

Figure 1.1 Large control system hierarchy
enterprise
Group Control
Unit Control
Field
Sensors
& Actors
A V
Supervisory
Primary
technology
Workflow, Resources, Interactions
SCADA
= Supervisory
Control And Data
Acquisition
T
administration Planning, Statistics, Finances
supervision
1
2
3
4
0

Distributed Control System and Programmable Logic Control
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1.1.2 Response Time and Hierarchical Level

Figure 1.2 Response Time And Hierarchical Level

1.2 What is DCS?

 A DCS is an integrated set of modules with distributed functions.
– Multi-loop controllers (10’s-100’s) that connect to field
devices
– Supervisory coordinating controllers
– Multi-loop operator stations and engineering stations
– Servers for system data management
– Control network for intercommunication
– External connections
Planning
Level
Execution
Level
Control
Level
Supervisory
Level
ms seconds hours days weeks month years

ERP
(Enterprise Resource
Planning)
DCS
MES
(Manufacturing
Execution System)
PLC
(Programmable
Logic Controller)
(Distributed
Control System)
(Supervisory Control
and Data Acquisition)
SCADA

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Figure 1.3 DCS Hierarchy

 A DCS, throughout the whole system, must provide:
– Performance: control must be faster than the process.
– Determinism: control must always take the same time.
– Fault tolerance: redundancy; must fail to a known state.
– Security: must have access restrictions/controls.

Even though performance, ease of use, and interoperability are key
evaluation criteria for any control system software package, the following
is intended to provide the manufacturing engineer with a concise list of
control system software evaluation criteria.
1. INTEROPERABILITY.
This refers to the interaction of all control system hardware and
software components at all levels.
2. INTERCONNECTIVITY.
This criterion is concerned with the transmission medium, which is
constrained by the network topology and how efficiently the
system’s components communicate with each other.
Remote
Server
Remote Users
System
Server
Operator
Stations
Supervisory
Controller
Engineering
Station
www
Multi-loop
Controller
Direct I/O Module
Other Industrial Devices
Control Network

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3. DISASTER PROCESSING.
This component is defined by the efficiency with which the
software provides the operator with system failure information and
the ease at which the operator is permitted to bring the system back
to maximum operation after system failure.
4. DATABASE.
This refers to the software’s ability to maintain the system’s
database.
5. PROCESSES/DATA.
This criterion is concerned with the variety of processes and data
that can be controlled by the SCADA package.
6. DIAGNOSTICS.
The SCADA package’s ability to assist in the resolution of system
failures is evaluated by this diagnostic utility.
7. SECURITY.
This component is concerned with the levels of security provided
by the software.
8. MONITORING/CONTROL
Monitoring of a given process in real-time and control of that
process, within preset parameters, is evaluated by this criteria.
9. ALARM MANAGEMENT/LOGGING.
This is the category for detecting, annunciating, managing, and
storing alarm conditions.
10. STATISTICAL PROCESS CONTROL.
This is the portion of the SCADA package that evaluates the
process data. Production and quality is greatly effected by this data.
12. OPERATOR INTERFACE.
The graphical user interface (GUI) is evaluated using this criterion.

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13. TRENDING.
The software’s ability to display trending plots using historical and
current data is considered in this category.
14. REPORT GENERATION.
The production of logs and reports using current real-time data and
data retrieved from historical files is evaluated under this category.

Due to the advancements in computer technology and low cost, a
personal computer-based distributed control system can be installed for a
fraction of the cost required just a few years ago. However, prior to
selecting any piece of DCS equipment, first examine the existing
equipment, in particular the smart controllers, for network compatibility.
Then, examine and select the software to be employed.

1.3 What is PLC?

A programmable logic controller, also called a PLC or
programmable controller, is a computer-type device used to control
equipment in an industrial facility. The kinds of equipment that PLCs can
control are as varied as industrial facilities themselves. Conveyor
systems, food processing machinery, auto assembly lines…you name it
and there’s probably a PLC out there controlling it.
In a traditional industrial control system, all control devices are
wired directly to each other according to how the system is supposed to
operate. In a PLC system, however, the PLC replaces the wiring between
the devices. Thus, instead of being wired directly to each other, all
equipment is wired to the PLC. Then, the control program inside the PLC
provides the “wiring” connection between the devices.

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The control program is the computer program stored in the PLC’s
memory that tells the PLC what’s supposed to be going on in the system.
The use of a PLC to provide the wiring connections between system
devices is called soft-wiring.
Let's say that a push button is supposed to control the operation of
a motor. In a traditional control system, the push button would be wired
directly to the motor. In a PLC system, however, both the push button and
the motor would be wired to the PLC instead. Then, the PLC's control
program would complete the electrical circuit between the two, allowing
the button to control the motor.

Figure 1.4 PLC development

A PLC basically consists of two elements:
 The central processing unit
 The input/output system

1.3.1 The Central Processing Unit

The central processing unit (CPU) is the part of a programmable
controller that retrieves, decodes, stores, and processes information. It
also executes the control program stored in the PLC’s memory. In

Distributed Control System and Programmable Logic Control
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essence, the CPU is the “brains” of a programmable controller. It
functions much the same way the CPU of a regular computer does, except
that it uses special instructions and coding to perform its functions. The
CPU has three parts:
 The processor
 The memory system
 The power supply
The processor is the section of the CPU that codes, decodes, and
computes data. The memory system is the section of the CPU that stores
both the control program and data from the equipment connected to the
PLC. The power supply is the section that provides the PLC with the
voltage and current it needs to operate.

Figure 1.5 Microprocessor Hardware

1.3.2 The input/output (I/O) system

It is the section of a PLC to which all of the field devices are
connected. If the CPU can be thought of as the brains of a PLC, then the
I/O system can be thought of as the arms and legs. The I/O system is what
actually physically carries out the control commands from the program
stored in the PLC’s memory.

Distributed Control System and Programmable Logic Control
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The I/O system consists of two main parts:
 The rack
The rack is an enclosure with slots in it that is connected to the CPU.
 I/O modules
I/O modules are devices with connection terminals to which the field
devices are wired.

Together, the rack and the I/O modules form the interface between the
field devices and the PLC. When set up properly, each I/O module is both
securely wired to its corresponding field devices and securely installed in
a slot in the rack. This creates the physical connection between the field
equipment and the PLC. In some small PLCs, the rack and the I/O
modules come prepackaged as one unit.

Figure 1.6 I/O Racks

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1.4 How is a DCS different from a PLC system?

DCS PLC
Mfr sells a complete system of integrated
components.
Mfr sells some components; an SI
acquires others and engineers the system.
Mfr supports the system. Mfr supports the components.
On-line repair/ maintenance are the norm. Off-line repair/ maintenance are the norm.
System management built-in. System management designed per project.
Users expect to evolve/upgrade/expand a
system over 10/20/30 years.
System is a one-off project (like a house).
Upgrades / expansions are new projects.

1.5 Redundancy and Fault Tolerance

1.5.1 Redundancy
 Hardware redundancy
– add extra hardware for detection or tolerating faults
 Software redundancy
– add extra software for detection and possibly tolerating faults

1.5.2 Fault Tolerance
 Error Detection
 Damage Confinement
 Error Recovery
 Fault Treatment

1.5.2.1 Error Detection
 Ideal check
– Check should be independent from system
– Check fails if system crashes

Distributed Control System and Programmable Logic Control
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 Acceptable check
– Cost
– Reasonable check, e.g. monitor rate of change
 diagnostics
– Performed “by system on system components”
– E.g. power-up diagnostics

1.5.2.2 Damage Confinement
 Error might propagate and spread
 Identify boundaries to state beyond which no information exchange
has occurred

1.5.2.3 Error Recovery
 Backward recovery
– State is restored to an earlier state
– Requires checkpoints
– Most frequently used
– Recovery overhead
 Forward recovery
– Try to make state error-free
– Need accurate assessment of damage
– Highly application-dependent

1.5.2.4 Fault Treatment
 If transient fault: restart system, goto error-free state
 System repair
– On-line, no manual intervention, (automatic)
– Dynamic system reconfiguration
– Spare (hot or cold)

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1.5.2.5 Fault Coverage
 Measure of system’s ability to perform:
– Fault detection
– Fault location
– Fault containment
– (and/or fault recovery)
 Note:
– Recovery implies that the system as a whole is operational
– This does not imply that a “repair” occurred
– E.g. duplex system with benign fault can recover to continue
operation on one non-faulty processor

1.5.2.6 Hardware Redundancy
 Passive (static)
– Uses fault masking to hide occurrence of fault
– No action from the system is required
– E.g. voting
 Active (dynamic)
– Uses comparison for detection and/or diagnoses
– Remove faulty hardware from system => reconfiguration
 Hybrid
– Combine both approaches
– Masking until diagnostic complete
– Expensive, but better to achieve higher reliability

1.5.2.7 Passive Hardware Redundancy
 N-Modular Redundancy (NMR)
– N independent modules replicate the same function

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 Parallelism
– Results are voted on requirements: N >= 3
 TMR (Triple Modular Redundancy)

1.5.2.8 Fault tolerant structures
Fault tolerance allows continuing operation in spite of a limited
number of independent failures. Fault tolerance relies on work
redundancy.

1.5.2.9 Static redundancy: 2 out of 3
 Workby of 3 synchronised and identical units.
– All 3 units OK: Correct output.
– 2 units OK: Majority output correct.
– 2 or 3 units failure: Incorrect output.
– Otherwise: Error detection output.

Figure 1.7 (2 out of 3) Redundancy
1.5.2.10 Dynamic Redundancy
 Redundancy only activated after an error is detected.
– Primary components (non-redundant)
– Reserve components (redundancy), standby (cold/hot standby)
sync
Voter
sync
Process input
Process output

Distributed Control System and Programmable Logic Control
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Figure 1.8 Dynamic Redundancy
1.5.2.11 Workby and Standby

Figure 1.9 Workby and Standby
1.5.2.12 Workby Fault-Tolerance for Integrity and Persistency

Figure 1.10 Workby Fault-Tolerance for Integrity and Persistency
Primary unit Standby unit
Switch
Output
Input
on-line workby
sync
= ?
on-line standby
sync
Hot standby Cold standby
Both computers are doing
the same calculations
at the same time
Comparison for easy
error detection.
Comparator needed.
Non-redundant continuation
in case of failure?
Standby is not computing
Error detection needed.
Easy switchover in case
of failure.
Easy repair of reserve unit.
Standby is no operational
Error detection needed.
Long switchover period
with loss of state info.
No aging of reserve unit.
Workby
disjunctor
comparator
INTEGER
Worker
synchronization
Matching
input
Co
-

Worker
Output
output
Worker
commutator
synchronization
Matching
PERSISTENT
input
Co
-

Worker
Output
output
E
D
E
D

Distributed Control System and Programmable Logic Control
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1.5.2.13 Hybrid Redundancy
Mixture of workby (static redundancy) and standby (dynamic redundancy).

Figure 1.11 Hybrid Redundancy

1.6 Microprocessor Control

For simple programming the relay model of the PLC is sufficient.
As more complex functions are used the more complex VonNeuman
model of the PLC must be used. A computer processes one instruction at
a time. Most computers operate this way, although they appear to be
doing many things at once. Consider the computer components shown in
Figure 1.12.

Figure 1.12 Simplified Personal Computer Architecture
voter
work-
by
work-
by
work-
by
stand-
by
stand-
by
voter
work-
by
failed
work-
by
work-
by
stand-
by
Reconfiguration
(self-purging
redundancy)

Distributed Control System and Programmable Logic Control
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Input is obtained from the keyboard and mouse, output is sent to the
screen, and the disk and memory are used for both input and output for
storage. (Note: the directions of these arrows are very important to
engineers, always pay attention to indicate where information is flowing.)
This figure can be redrawn as in Figure 1.13 to clarify the role of inputs
and outputs.

Figure 1.13 An Input-Output Oriented Architecture

In this figure the data enters the left side through the inputs. (Note:
most engineering diagrams have inputs on the left and outputs on the
right.) It travels through buffering circuits before it enters the CPU. The
CPU outputs data through other circuits. Memory and disks are used for
storage of data that is not destined for output. If we look at a personal
computer as a controller, it is controlling the user by outputting stimuli on
the screen, and inputting responses from the mouse and the keyboard.
A PLC is also a computer controlling a process. When fully
integrated into an application the analogies become;
 Inputs - the keyboard is analogous to a proximity switch input
circuits - the serial input chip is like a 24Vdc input card

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 Computer - the 686 CPU is like a PLC CPU unit
 Output circuits - a graphics card is like a triac output card
 Outputs - a monitor is like a light
 Storage - memory in PLCs is similar to memories in personal
computers

It is also possible to implement a PLC using a normal Personal Computer,
although this is not advisable. In the case of a PLC the inputs and outputs
are designed to be more reliable and rugged for harsh production
environments.

1.7 Role Play

Each trainee should act a role play on the following:
1. Automation system structure.
2. What DCS and PLC and their differences?
3. Redundancy and fault tolerance.

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Chapter 2
Regulatory Control

2.1 Learning Objectives

 Introduce Regulatory Control.
 Understanding PID control.
 Differentiate between various control loops.

2.2 Introduction

Most of the applications of industrial control process used simple
loops which regulated flows, temperatures, pressures and levels.
Occasionally ratio and cascade control loops could be found. There are
many benefits for using regulatory control. One of the most important is
simply closer control of the process. Process control is one part of an
overall control hierarchy that extends downwards to safety controls and
other directly connected process devices, and upward to encompass
process optimization and even higher business levels of control such as
scheduling, inventory management.

Most control engineers would recognize the form of response
shown in figure 2.1. Actually the response could be determined by
solving a differential equation. It is more important to have a good
understanding of the physical response than to be able to predict the
solution by solving the differential equation.

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Figure 2.1 Response of simple dynamic process to step input change
Instrumentation, control and process engineers abstract the pictorial
form of the process into an iconographic diagram called "Piping and
Instrumentation Diagram", i.e. P&ID. Figure 2.2 is an example of the
P&ID.

Figure 2.2 Control loop representation used on P&IDs.
For description and analysis of a control loop, without referring to
whether it is implemented with analog or digital hardware, a block
diagram as shown in figure 2.3 is beneficial.

Figure 2.3 Simplified block diagram representation of process control loop.

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2.3 PID Control

2.3.1 Feedback Control

The principle of feedback is one of the most intuitive concepts
known. An action is taken to correct a less satisfactory situation then the
results of the action are evaluated. If the situation is not corrected then
further action takes place. Feedback control can be classified by the form
of the controller output. One of the simplest forms of output is discrete
form, also called on-off or two position control. An example of this is the
household thermostat, which activates heating unit if the temperature is
below the setting, or deactivates the unit if the temperature is above the
setting.

Figure 2.4 On-Off Control.
The idea of two position control can be extended to multi-position
control; an example is commercial air-conditioning refrigeration
equipment which is operated by loading and unloading compressor
cylinders. The ultimate extension is infinite number of positions which is
called modulating control; an example is the process controller output
that can drive a valve to any position between 0 and 100 percent, as
shown in figure 2.5.

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Figure 2.5 Flow versus position, infinite position Control.

2.3.2 Modes of Control

Feedback controllers use one, two, or three methods to determine
the controller output. These methods, called the modes of control,
including the following:
 Proportional (P)
 Integral (I)
 Derivative (D)
In general these modes can be used singly or in combination.

2.3.2.1 Proportional Mode

With a controller containing only the proportional mode, the
controller output is proportional to the measurement value only. Neither
history of the measurement value nor consideration to the rate of change
is utilized. Adjustment, i.e. tuning, of the controller is simple because
there is only one adjustment as shown in figure 2.6.

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Figure 2.6 Relationship between input and output for proportional control.
Figure 2.7 illustrates a proportional control system. The rate of
fluid flow into the tank represents the load. To be in equilibrium, the
outflow must be the same as the inflow. The outflow is achieved by a
particular valve position where the fixed mechanism between the float,
pivot and link attain.

Figure 2.7 Proportional control.

2.3.2.2 Integral Mode
An integrator is the ideal device for automating the procedure for
adjusting the controller output bias. It is called the automatic reset.

2.3.2.3 Derivative Mode
The derivative is used to anticipate the effect of load changes by
adding a component to the controller output that is proportional to the rate
of change of the measurement. See figure 2.8.

Distributed Control System and Programmable Logic Control
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Figure 2.8 PID control.

2.3.3 Control Loop Structure

For microprocessor control system, control strategy is configured
by a series of software function blocks. Just like a set of hardware
modules require interconnections to form a complete control system, a set
of software function blocks also acquire interconnections, i.e. soft-wiring.

Figure 2.9 shows a simple feedback loop with the software portion
consists of three function blocks:
 An analog input block that causes the analog to digital converter to
convert the incoming 4-20mA signal to an analogous value. The value
is deposited in a memory register.
 A PID control block which obtains the measurement value from the
analog input block and compares it with the setpoint then it executes a
PID algorithm to calculate the output.
 An analog output block that obtains from the PID block the required
valve position value. The value is converted by a digital to analog
converter to 4-20mA signal.

Distributed Control System and Programmable Logic Control
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Figure 2.9 Control loop hardware/software structure.

2.3.4 Control Loop Tuning

The power of PID control is that by good choice of control
parameters the controller can be adjusted to provide the desired behavior
on a wide variety of process applications. Determining acceptable values
of these parameters is called tuning the controller. A good criterion for
acceptable performance is the "quarter cycle decay" shown in figure 2.10.

Figure 2.10 quarter cycle decay criterion

Most loops are tuned by experimental techniques, i.e. trial and error.
Figures 2.11 and 2.12 give a tuning map for adjusting control parameters.

Distributed Control System and Programmable Logic Control
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Figure 2.11 Gain and Reset effects.

Figure 2.12 Derivative effects.
2.4 Control Loop Types
2.4.1 Ratio Control

Figure 2.13 shows the P&ID of a process heater in which the fuel
flow is measured and multiplied by the required air-to-fuel ratio; this
results in the required air flow rate, which is introduced as a setpoint of
the feedback controller. The required air-to-fuel ratio is automatically
adjusted as the output of the stack O2 controller.

Distributed Control System and Programmable Logic Control
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Figure 2.13 ratio Control..
2.4.2 Cascade Control

In figure 2.14 the temperature controller cascades a steam flow
controller. The temperature controller would react to outlet temperature
drop by increasing the setpoint of the steam flow controller, which in turn
would increase the signal to the valve. The flow will quickly respond to
increased demand from the temperature controller and thus reaching the
desired setpoint of the outlet temperature stream.

Figure 2.13 Cascade Control.
2.4.3 Feedforward Control

With feedforward control, the objective is to drive the controlling
device from a measurement of the disturbance that is affecting the
process, rather than from the process variable itself. In figure 2.14, the

Distributed Control System and Programmable Logic Control
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application was analyzed the variation in process inlet temperature was
the principle of disturbance. Hence, a feedforward controller is used to
drive the fuel flow controller by sensing the inlet temperature.

Figure 2.14 Feedforward Control.
2.4.4 Selector (Override) Control

There are several ways of using selector switches in control
strategies. One way is to select the higher (or lower) of several
measurement signals to pass the process variable to a feedback controller.
For example, the highest of several process temperatures may be selected
automatically to become the controlling temperature as shown in figure
2.15.

Figure 2.15 Override Control.

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2.4.5 Split Range Control

Split range control when one process variable such as plant inlet
pressure is used to manage two different output devices such as plant
bypass control valve and flow control loop for fractionation area. The 4-
12 mA signal is used to control the flow control loop. If the plant cannot
handle all incoming feed, the 12-20 mA signal control the plant bypass
valve to direct extra feed to the outside of the plant.

2.5 Role Play

The trainees are required to play roles about:
1. Introducing regulatory control.
2. Introducing modes of control.
3. Intruding control loop types.

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Distributed Control System and Programmable Logic Control
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Chapter 3
DCS Infrastructure

3.1 Learning Objectives

 Introduce system infrastructure interoperability and interconnectivity.
 Illustrate system components of level 2 control.

3.2 Communication Bus

Figure 3.1: Communication Bus
The communication bus, i.e. the Nodebus, interconnects stations
(Control Processors, Application Processors, Application Workstations,
and so forth) in the system to form a process management and control
node. Depending on application requirements, the node can serve as a
single, stand-alone entity, or it can be configured to be part of a more
extensive communications network.
Operating in conjunction with the Nodebus interface electronics in
each station, the Nodebus provides high-speed, redundant, peer-to-peer
communications between the stations.
The high speed, coupled with the redundancy and peer-to-peer
characteristics, provide performance and security superior to that

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provided by communication media used in conventional computer-based
systems. Station interfaces to the Nodebus are also redundant, further
ensuring secure communications between the stations. The Nodebus can
be implemented in a basic, non-extended configuration or it can be
extended through the use of Nodebus Extenders and Dual Nodebus
Interface Extenders.

3.2.1 Nodebus Interface

The Nodebus Interface is a module which allows direct connection
of a personal workstation (PW), with appropriate Nodebus connector card
and software, to the Nodebus figure 3.2. In this configuration, the PW
functions as a station on the node. The Nodebus Interface allows
connection of a station application workstation hosting an Ethernet
configuration to Nodebus. See figure 3.2.

Figure 3.2 Nodebus Interface Implementation (Typical)

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An Attachment Unit Interface (AUI) cable, connects the PW or an
Ethernet hub configuration to the Nodebus via a Nodebus Interface. A
coaxial cable (ThinNet) connects an Ethernet daisy chain configuration to
the Nodebus via a Nodebus Extender. The Nodebus Interface is non-
redundant, and can be used in any of the Nodebus configurations
described.

3.2.2 Dual Nodebus Interface

The Dual Nodebus Interface (DNBI) is a module which allows
direct connection of stations to the appropriate Nodebus. Connection
between the DNBI and station is made via an AUI cable.

For data transmission security, a separate (RS-423) control cable
connects between the station and the DNBI to allow switching between
the two redundant Nodebus cables. Switching of the Nodebus cables is
controlled by the station, which transmits commands to the DNBI via the
control cable. Figure 3.3 shows connection of a station to the Nodebus
using a DNBI.

Figure 3.3 Local Connection of Station

3.2.3 Dual Nodebus Interface Extender

The Dual Nodebus Interface Extender (DNBX) is functionally
similar to the DNBI, but provides a greater cabling distance. The

Distributed Control System and Programmable Logic Control
37
principal transmission medium used is a coaxial Ethernet cable directly
connected to the station end by a standard Ethernet transceiver. Figure 3.4
remote connection of a station to the Nodebus using a DNBX.

Figure 3.4 Remote Connection of Station

3.3 Control Processor

The Control Processor performs regulatory, logic, timing, and
sequential control together with connected:
 Fieldbus Modules (FBMs)
 Fieldbus Cluster I/O Cards (FBCs)
It also performs data acquisition (via the Fieldbus Modules), alarm
detection and notification, and may optionally serve as an interface for
one or more Panel Display Stations.
The non-fault-tolerant version of the Control Processor is a single-width
processor module. The fault-tolerant version consists of two single-width
processor modules.

3.3.1 Enhanced Reliability

The Control Processor offers optional fault- tolerance for enhanced
reliability. The fault-tolerant control processor configuration consists of
two parallel-operating modules with two separate connections to the
Nodebus and to the Fieldbus.

Distributed Control System and Programmable Logic Control
38
The two control processor modules, married together as a fault-
tolerant pair, are designed to provide continued operation of the unit in
the event of virtually any hardware failure occurring within one module
of the pair. Both modules receive and process information
simultaneously, and the modules themselves detect faults. One of the
significant methods of fault detection is comparison of communication
messages at the module external interfaces. Upon detection of a fault,
self-diagnostics are run by both modules to determine which module is
defective. The non-defective module then assumes control without
affecting normal system operations.

To further ensure reliable communications, the fault-tolerant
control processor performs error detection and address verification tests
in its Nodebus and Fieldbus interfaces. For enhanced reliability during
maintenance operations, the Control Processor is equipped with a
recessed reset button. This feature provides for manually forcing a
module power off and on (reboot) without removing the module from the
enclosure.

3.3.2 Diagnostics

The Control Processor uses three types of diagnostic tests to detect
and/or isolate faults:
 Power-up self-checks
 Run-time and watchdog timer checks
 Off-line diagnostics
Power-up self-checks are self-initiated when power is applied to
the control processor. These checks perform sequential tests on the
various control processor functional elements. Red and green indicators at

Distributed Control System and Programmable Logic Control
39
the front of the control processor module reflect the successful (or non-
successful) completion of the various phases of the control processor
startup sequence.
The run-time and watchdog timer checks provide continuous
monitoring of control processor functions during normal system
operations. The operator is informed of a malfunction by means of
printed or displayed system messages.
Off-line diagnostics are temporarily loaded into the system for the
purpose of performing comprehensive tests and checks on various system
stations and devices. Using the off-line diagnostics, a suspected fault in
the control processor can be isolated and/or confirmed.

3.4 Engineering Interface

The engineering interface, i.e. Application Processor, is
microprocessor-based application processor/file server stations. They
perform two basic functions:
 As application processor (computer) stations, they perform
computation intensive functions.
 As file server stations, they process file requests from tasks within
themselves or from other stations. Bulk storage devices used with
the Application Processors include floppy disk drives, hard disk
drives, streaming tape drives, and CD-ROMs.

The Application Processors operate in concert with other system
stations (such as communication processors, workstation processors, and
control processors), which provide the necessary means for data
input/output and operator interfacing. A smaller system can utilize a
single Application Processor, while a larger system can incorporate

Distributed Control System and Programmable Logic Control
40
several Application Processors, each configured to perform specific
functions. Some functions can be performed by individual Application
Processors, while others can be shared by two or more Application
Processors in the same network.
For all models of the Application Processor, applications range
from minimal functions, such as the storage of memory images, alarm
events, and historical data, to larger-scale applications such as database
management and program development.

3.4.1 Application Processor Functions

The following sections describe the major functions performed by
the Application Processors.

3.4.1.1 System and Network Management Functions

The Application Processors perform system management
functions, which include collecting system performance statistics, data
reconciliation, performing station reloads, providing message
broadcasting, handling all station alarms and messages, and maintaining
consistent time and date in all system stations. The Application Processor
also performs network management functions, which comprise that
portion of system management functions which deal with the network.

3.4.1.2 Database Management

Database management involves the storage, manipulation, and
retrieval of files containing data received and/or produced by the system.
The Application Processors utilize the industry-standard Relational Data
Base Management System.

Distributed Control System and Programmable Logic Control
41
3.4.1.3 File Requests

Each Application Processor contains a file manager, which
manages all file requests associated with bulk memory attached to the
Application Processor. Each Application Processor also supports a
remote file system that allows tasks in one station to share files in
another.

3.4.1.4 Historical Data

The Application Processors can be configured to contain the
Historian function, which maintains a history of application messages and
continuous and discrete I/O values. These values may represent any
parameters such as measurements, setpoints, outputs, and status switches
from stations that have been configured to collect data and send it to a
Historian. In addition, the Historian computes and stores a history of
averages, maximums, minimums, and other derived values. This
information is maintained for display, reporting, and access by
application programs. An archiving facility saves the data on removable
media, where applicable.

The Application Processors can be configured to maintain a history
of errors, alarm conditions, and selected operator actions. The occurrence
of errors, alarms, and events in other stations can be stored (for later
review and analysis) by sending a message defining the event to the
Historian in one or more Application Processors.

Distributed Control System and Programmable Logic Control
42
3.4.1.5 Graphic Display Support

The Application Processor supports graphic displays by storing and
retrieving display formats, by providing access to objects stored on the
Application Processor, and by storing tasks which execute in a
workstation processor. Application Processors not only provide storage of
information and file management for displays, but also execute programs
that perform display and trend service.

3.4.1.6 Production Control Software

Production control software represents a large range of packages
that require varied Application Processor resources. The following is a
list of packages provided:
 DBMS
 Historian
 Spreadsheet
 Physical Properties Library
 Mathematics Library
 BATCH
The operation and performance of the production control software are
determined by the particular Application Processor configuration.

3.4.1.7 Configuration

Configuration refers to the process of entering or selecting
parameters to define what a software package does, or to define the
environment for a software package. The Application Processors support
configuration functions by providing bulk storage for configuration
parameters and by executing some of the configuration processes.

Distributed Control System and Programmable Logic Control
43
3.4.1.8 Application Development Facilities

Application development tools are provided to build programs for
all system stations. These include tools to document, enter, translate, link,
test, and maintain programs written in several programming languages.
The Application Processor supports program development for all stations
(workstation processors, control processors, and so forth).
Assembly language, FORTRAN, and C programs can be written on
the Application Processor using standard operating system tools. An
optional package is available including text editors, debuggers, linkers,
revision control, and compilers, plus execution statistics functions.

3.4.1.9 User Application Program Execution

The Application Processors also execute user application programs.
These may be application packages such as special optimizations, test
data collections, special data reductions, or other packages that you may
have already developed. The allocation of resources reserved for user
application varies with each Application Processor.

3.4.2 Diagnostics

The Application Processors utilize three types of diagnostic tests to
detect and/or isolate faults:
 Power-up self-checks
 Run-time and watchdog timer checks
 Off-line diagnostics
Power-up self-checks are initiated when power is applied to the
Application Processor. These checks perform sequential tests on the

Distributed Control System and Programmable Logic Control
44
various Application Processor functional elements. Any malfunction
detected during the power-up self-checks is reported by means of
messages printed or displayed on a directly connected printer or terminal.
The run-time and watchdog timer checks provide continuous
monitoring of Application Processor functions during normal system
operations. For any processor model, you are informed of a malfunction
by means of printed or displayed system messages. Off-line diagnostics
are temporarily loaded into the system for the purpose of performing
comprehensive tests and checks on various system stations and devices.
Using the off-line diagnostics, a suspected fault in the Application
Processor can be isolated and/or confirmed.

3.4.3 Workstation Components

The workstation components provide user interface to all System
CRT display functions. A selection of workstation components is
available for command and data entry, along with CRT pointer
manipulation and control. These components interact with software
resident in versions of the system Workstation Processors (WPs) and
Application Workstation Processors (AWs). Many of these components
(displays and keyboards) are "common" and allow interchangeability and
simplicity in mixed technology configurations.
Workstation components include:
 Alphanumeric Keyboard
 Annunciator and Annunciator/Numeric Keyboards
 Workstation Display (with/without Touchscreen)
 Mouse
 Trackball
 Industrial Pointing Device

Distributed Control System and Programmable Logic Control
45
 Workstation Processor or Application Workstation Processor
 Personal Workstation
 Modular Industrial Console
Selection of the touch screen, mouse, trackball or industrial
pointing device is required for picking display objects on the CRT. The
touch screen has sufficient resolution for all functions normally
associated with a process operator. Only the mouse or trackball provides
the picking resolution necessary for engineer-related functions (for
example, building graphic displays). The touch screen associated with
Workstation Display and the annunciator type keyboards connects to a
Graphics Controller Input Output (GCIO) interface unit located beneath
the workstation display. The GCIO interfaces to the Workstation
Processor and/or Application Workstation that provide secure, high-
speed, bidirectional data flow. The alphanumeric keyboard and trackball
connect together in a functional grouping via a serial communications
link to the processors. Personal Workstations (PW) utilize separate serial
communication links for alphanumeric keyboard and mouse/trackball.
These buses allow a variety of component connections.

Figure 3.5 Table- Workstation Components.

Distributed Control System and Programmable Logic Control
46
3.4.3.1 Alphanumeric Keyboard

The alphanumeric keyboard is used any time text is entered into the
system. It consists of the full set of alphanumeric keys plus punctuation
and special symbol keys laid out in the standard format, and a numeric
data entry pad (with cursor control).

Figure 3.6 Alphanumeric Keyboard
3.4.3.2 Annunciator Keyboard

The Annunciator Keyboard Figure 3.7 is an array of LED/switch
pairs. It also contains a horn silence switch and a lamp-test switch. Each
LED, under control of the processor software, may be ON, OFF, or
FLASHING as determined by the process conditions. The LEDs, when
used in conjunction with the unit's audible annunciator, form an effective
means of calling a user's attention to specific areas of the system. The
switch associated with each LED can be used to invoke any pre-
configured displays or operator responses..

Figure 3.7 Annunciator Keyboard
3.4.3.3 Workstation Display with/without Touchscreen

The workstation display is an analog cathode ray tube (CRT) color
monitor supporting ultra-high resolution applications. The monitor is
suitable for mounting onto a Modular Industrial Workstation or on a

Distributed Control System and Programmable Logic Control
47
desktop. The monitor can include a touchscreen optional feature. Figure
3.8 shows the monitor with a tilt and swivel base mounted on the GCIO
interface unit. The GCIO interface supports the touchscreen, annunciator
and annunciator/ numeric keyboard, and audible horn options.

Figure 3.8 Table-Top Workstation Display
The optional touch screen is bonded to the front surface of the CRT
monitor. The user selects display objects by touching them on the screen.
The touch screen senses the action and sends a data signal to the
workstation processor's software indicating the position of the selection.
3.4.3.4 Trackball

The trackball is a stationary component that contains a rotatable
sphere. The trackball can be located on a table top. Rotation of the sphere
causes CRT pointer movement analogous to the mouse action. Buttons
are also provided for user selections/manipulations. See Figure 3.9

Figure 3.9 Trackball
3.4.3.5 Modular Industrial Console

Modular Industrial Consoles provide flexible mounting
arrangements of components. They allow users to configure centralized
or distributed control centers tailored to the functional requirements of
each interaction point in the plant. The modular console furniture

Distributed Control System and Programmable Logic Control
48
described herein may incorporate a mix of equipment - console displays,
input devices, processors, Fieldbus Modules, data storage devices, and so
on. Alternately, only display-specific equipment can be incorporated.
Modular Industrial Consoles (MICs) are ideal for supporting powerful
multiple-screen, real-time display software interactions. This combination
allows console resources to be optimally allocated to meet changing day-
to-day needs.

3.5 Operator Interface

Operating in conjunction with human interface input/output
components, the workstation processors serve as a link between the
operator and other distributed processor modules. They receive graphic
and textual information both stored internally or from application
processors and generate signals to display the information on a
workstation display. Display formats and data files are available from
bulk storage. Live display information (distributed data objects) is
available from any control -processor, or from shared system global data.
The video information displayed can include free form combinations of
text, graphic illustrations, charts, and control displays.
The workstation processors display textual information as 80 text
characters per line, with four fonts. The processors provide resizable and
restackable windows. Displays for all of the workstation processors may
also be developed using the system software running in a compatible
personal computer.
A workstation processor, together with its workstation monitor and input
components, can be configured with combinations of peripherals to suit
functions and user preferences.

Distributed Control System and Programmable Logic Control
49
3.6 Gateways

The architecture of the DCS permits it to be connected to other
foreign systems using a gateway module for adapting different
communication protocols. See figure 3.10.

Figure 3.10 Field Automation Subsystem

3.7 Role Play

Each trainee should introduce one of the main components:
1. Communication Bus
2. Control Processor.
3. Application Processor
4. Operator Interfaces and Gateways

Distributed Control System and Programmable Logic Control
50
Chapter 4
DCS Hardware

4.1 Learning Objectives

 Define fieldbus communication.
 Illustrate system components of level 1 control.
 Demonstrate interconnection between different components.
 Develop knowledge base of foundation fieldbus technology.

4.2 Fieldbus Modules

Fieldbus Modules provide connection of digital I/O, analog I/O,
and Intelligent Transmitters to control processors. There are two types of
Fieldbus Modules: Main and Expansion. Some main modules can be
expanded using an expansion module.

A wide range of Fieldbus Modules is available to perform the
signal conversion necessary to interface the control processor with field
sensors and actuators.

4.3 Fieldbus Interconnection

The Control Processor is used in three different configurations, which
provide broad flexibility in Fieldbus implementation:
 Local Fieldbus (Figure 4.1) - Used only within the enclosure.
Fieldbus Modules attach directly to the redundant local bus.

Distributed Control System and Programmable Logic Control
51

Figure 4.1 Local Fieldbus
 Twinaxial (Dual-Conductor Coaxial) Fieldbus Extension (Figure
4.2) - Using twinaxial cable, the Fieldbus can optionally extend
outside of the enclosure. Fieldbus Modules attach to the extended
bus through Fieldbus isolators. The twinaxial Fieldbus extension
may be redundant.

Figure 4.2 Twinaxial Fieldbus Extension
 Fiber Optic Fieldbus Extension (Figure 4.3) - The fiber optic
Fieldbus can optionally extend the distance as well as add
application versatility and security.

Figure 4.3 Fiber Optic Fieldbus Extension

Distributed Control System and Programmable Logic Control
52
All three Fieldbus configurations use serial data communication
complying with Electronic Industrial Association (EIA) Standard RS-485.

4.4 Cluster I/O Subsystem Interfacing

The Control Processor interfaces with the Fieldbus Cluster
Input/Output Subsystem that consists of the Fieldbus, a multi-slot chassis
configuration of a Fieldbus Processor, analog/digital Fieldbus Cards
(FBCs), and power supply and power monitor card. These Cluster I/O
subsystems meet the needs of applications where a high number of
channels per card are required. Figure 4.4 shows a typical twinaxial
Fieldbus configuration.

Figure 4.4 Twinaxial Fieldbus Cluster I/O Subsystem Interface Configuration
4.5 Fieldbus Cluster I/O Subsystem

The Fieldbus Cluster Input/Output Subsystem provides full support
for analog measurement, digital sensing, and analog or discrete control
capabilities. The Subsystem integrates with the Control Processor or
Personal Workstation via the Fieldbus, and includes a multi-slot chassis
configuration made up of a Fieldbus Processor, Analog/Digital Fieldbus
Cards (FBC), subsystem main power supply, and power monitor card.

Distributed Control System and Programmable Logic Control
53
The Fieldbus Cluster I/O Subsystem is configurable, gathering
analog measurements, while simultaneously handling analog and digital
input and output channels. The Fieldbus Cluster I/O Subsystem is offered
in both non-redundant and redundant configurations. Each in a redundant
pair is individually addressable on the Fieldbus with a unique logical
address. In a redundant configuration, the FBPs provide switchover from
the primary FBP to the redundant FBP and back again automatically. The
FBCs are suitable in applications where a high number of channels per
card are required. They are ideal for non-isolated and isolated input signal
gathering and data acquisition systems where high quantities of "points
per cluster" areas are desired. The FBCs may be optionally connected as
redundant pairs. Various input cards are available with one of the
following three levels of isolation:
 Non-isolated - Each channel is referenced to ground and the card
itself is referenced to ground.
 Group-isolated - Electrically separate card-to-card but not channel-
to-channel on the same card.
 Isolated - Each channel is electrically separated from any other
channel, card, group, building, site, etc.

4.6 Fieldbus Processor

The Fieldbus Processor (FBP) module provides communication
between the Fieldbus Cards (FBCs) and the Control Processor. Optionally
available is redundancy for the FBP module. Each FBP module is
individually addressable via the Fieldbus. If the primary FBP fails or is
taken off-line, the secondary FBP automatically assumes control. It
remains in control until the primary FBP returns on-line (figure 4.5).

Distributed Control System and Programmable Logic Control
54

Figure 4.5 FBP Overview
4.7 Fieldbus Cards

The Fieldbus Cards support a variety of analog and digital I/O
signals. The FBCs convert electrical I/O signals used by field devices to
permit communication with these devices via the Fieldbus.
The FBCs can be connected in a redundant configuration via the
hardware. The redundant FBCs must be in adjacent slots and they are
connected via a hardware adapter at the interface to the field devices. In
an FBC redundant configuration, the FBP determines which FBC of the
redundant pair is to supply the data to the Control Processor. This is done
in the software by a predetermined set of conditions.

4.7.1 Analog FBCS

The analog FBCs support analog signal types and control functions
equipped with accurate signal conditioning circuitry, the analog cards
interface between process sensors and actuators.
To input an analog voltage (into DCS) the continuous voltage value
must be sampled and then converted to a numerical value by an A/D

Distributed Control System and Programmable Logic Control
55
converter. Figure 4.6 shows a continuous voltage changing over time.
There are three samples shown on the figure. The process of sampling the
data is not instantaneous, so each sample has a start and stop time. The
time required to acquire the sample is called the sampling time. A/D
converters can only acquire a limited number of samples per second. The
time between samples is called the sampling period T, and the inverse of
the sampling period is the sampling frequency (also called sampling rate).
The sampling time is often much smaller than the sampling period.

Figure 4.6 Sampling an analog voltage
Analog outputs are much simpler than analog inputs. To set an
analog output an integer is converted to a voltage. This process is very
fast, and does not experience the timing problems with analog inputs.
But, analog outputs are subject to quantization errors. Figure 4.7 gives a
summary of the important relationships. These relationships are almost
identical to those of the A/D converter. Assume we are using an 8 bit D/A
converter that outputs values between 0V and 10V. We have a resolution
of 256, where 0 results in an output of 0V and 255 results in 10V. The
quantization error will be 20mV. If we want to output a voltage of
6.234V, we would specify an output integer of 159, this would result in
an output voltage of 6.235V. The quantization error would be 6.235V-
6.234V=0.001V. The current output from a D/A converter is normally
limited to a small value, typically less than 20mA.

Distributed Control System and Programmable Logic Control
56

Figure 4.7 D/A converter

4.7.2 Digital FBCS

The digital FBCs consist of 32- and 64-channel types. Inputs can
be either voltage monitoring or contact sensing.
Contact inputs must convert a variety of logic levels to the 5Vdc
logic levels used on the data bus. This can be done with circuits similar to
figure 4.8. Basically the circuits condition the input to drive an
optocoupler. This electrically isolates the external electrical circuitry from
the internal circuitry. Other circuit components are used to guard against
excess or reversed voltage polarity.

Figure 4.8 Contact input circuitry.
Contact outputs must convert the 5Vdc logic levels on the DCS
data bus to external voltage levels. This can be done with circuits similar
to figure 4.9. Basically the circuits use an optocoupler to switch external
circuitry. This electrically isolates the external electrical circuitry from

Distributed Control System and Programmable Logic Control
57
the internal circuitry. Other circuit components are used to guard against
excess or reversed voltage polarity.

Figure 4.9 Contact output circuitry.

4.8 Other Modules

 0 to 20 mA Input/Output Interface
 Pulse Input, 0 to 20 mA Output Interface
 Thermocouple/ Millivolt Input Interface
 RTD Input Interface
 High Power Contact/dc Input/Output Interface

4.9 Foundation Fieldbus Technology

FOUNDATION fieldbus is an all-digital, serial, two-way
communications system that serves as the base-level network in a plant or
factory automation environment.

Figure 4.10 Foundation Fieldbus Network

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58

Figure 4.11 Historical development of field devices technology.

It's ideal for applications using basic and advanced regulatory
control, and for much of the discrete control associated with those
functions. Two related implementations of FOUNDATION fieldbus have
been introduced to meet different needs within the process automation
environment. These two implementations use different physical media
and communication speeds.
 H1 works at 31.25 Kbit/sec and generally connects to field devices.
It provides communication and power over standard twisted-pair
wiring. H1 is currently the most common implementation and is
therefore the focus of these courses.
 HSE (High-speed Ethernet) works at 100 Mbit/sec and generally
connects input/output subsystems, host systems, linking devices,
gateways, and field devices using standard Ethernet cabling. It
doesn't currently provide power over the cable, although work is
under way to address this.

Figure 4.12 Field Device Capacity.

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59
Conventional analog and discrete field instruments use point-to-
point wiring: one wire pair per device. They're also limited to carrying
only one piece of information -- usually a process variable or control
output -- over those wires. As a digital bus, FOUNDATION fieldbus
doesn't have those limitations.

 Multidrop wiring. FOUNDATION fieldbus will support up to 32
devices on a single pair of wires (called a segment) -- more if
repeaters are used. In actual practice, considerations such as power,
process modularity, and loop execution speed make 4 to 16 devices
per H1 segment more typical.

That means if you have 1000 devices -- which would require 1000 wire
pairs with traditional technology -- you only need 60 to 250 wire pairs
with FOUNDATION fieldbus. That's a lot of savings in wiring (and
wiring installation).

Figure 4.12 Fieldbus wiring diagram.

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60
 Multivariable instruments. That same wire pair can handle
multiple variables from one field device. For example, one
temperature transmitter might communicate inputs from as many as
eight sensors -- reducing both wiring and instrument costs.

Other benefits of reducing several devices to one can include fewer pipe
penetrations and lower engineering costs.

 Two-way communication. In addition, the information flow can
now be two-way. A valve controller can accept a control output
from a host system or other source and send back the actual valve
position for more precise control. In an analog world, that would
take another pair of wires.
 New types of information. Traditional analog and discrete devices
have no way to tell you if they're operating correctly, or if the
process information they're sending is valid.

But FOUNDATION fieldbus devices can tell you if they're operating
correctly, and if the information they're sending is good, bad, or
uncertain. This eliminates the need for most routine checks -- and helps
you detect failure conditions before they cause a major process problem.

 Control in the field. FOUNDATION fieldbus also offers the
option of executing some or all control algorithms in field devices
rather than a central host system. Depending on the application,
control in the field may provide lower costs and better performance
-- while enabling automatic control to continue even if there's a
host-related failure.

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61
FOUNDATION fieldbus is covered by standards from three major
organizations:
 ANSI/ISA 50.02
 IEC 61158
 CENELEC EN50170:1996/A1

The technology is managed by the independent, not-for-profit
Fieldbus Foundation, whose 150+ member companies include users as
well as all major process automation suppliers around the globe.
Some suppliers have even donated fieldbus-related patents to the Fieldbus
Foundation to encourage wider use of the technology by all Foundation
members.

Interoperability simply means that FOUNDATION fieldbus
devices and host systems can work together while giving you the full
functionality of each component.

4.10 Role Play

Each trainee should introduce one of the main components:
5. Fieldbus Module and Interconnection
6. Fieldbus Processor and Clusters.
7. Foundation Fieldbus technology

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Chapter 5
DCS Software

5.1 Learning objectives

 To be familiar with main software components of DCS.
 Understand main tasks for each application.

5.2 Standard Application Packages
5.2.1 System Management
Features include:
 Display of equipment information for the station and its associated
input/output devices, buses, and printers.
 Capability for change actions directed to the associated equipment.
 Processing of station alarm conditions and messages.
5.2.2 Database Management
Features include:
 Storage, retrieval, and manipulation of system data files.
 A run-time license for the embedded use of the Relational Database
Management System.
 A spreadsheet package.

5.2.3 Historian
Features include:
 Maintenance of a history of values for process-related
measurements that have been configured for retention by the
Historian.

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63
 Maintenance of a history of application messages that have been
sent to the Historian.
 Maintenance of a history of alarms and error conditions which
generate messages for the Historian.
 Access to all Historian data by display and report application
programs.

5.2.4 View Display Manager
Features include:
 Presentation of the operating environment.
 Setting of the overall operating environment according to the type
of user. Process engineers, process operators, and software
engineers have access to specialized functions and databases suited
to their specific requirements and authorizations.
 Dynamic and interactive process graphics.
 Display and processing of current process alarms.
 Group and default displays for control blocks.
 Execution of embedded trending within displays.

5.2.5 Draw Display Builder
Features include:
 Graphical display configuration for viewing and control of process
operation.
 Access to graphical object palettes allowing easy inclusion of
pumps, tanks, valves, ISA symbols, and similar complex objects.
 Ready modification of existing displays using a mouse pointer,
menu items, and quick-access toolbars.
 Association of process variables with objects in the displays.

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64
 Dynamic variation of object attributes such as fill level, color,
position, size and visibility with changes in the associated process
variable.
 Inclusion of operator control elements such as pushbuttons and
sliders into displays.
 A library of faceplates which may be configured by simply
specifying the compound and block name of the block to which the
faceplate is to be connected.

5.3 Alarm System

Figure 5.1 Alarm manger
Alarm Manager provides an easy-to-use graphical interface of
preconfigured alarm displays for viewing and quickly responding to
process alarm conditions. The alarm display windows present alarm
messages initiated by the control blocks and related to digital input, state
change, absolute analog, deviation, rate of change, device status
mismatch, and other alarm conditions.
Accessible from any environment, the Alarm Manager Display windows
provide:

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65
 Quick, easy access to the most recent alarm messages via the Most
Recent Alarm display or Current Alarm display
 Alarm status and value information dynamically updated from the
control station
 Color-coded priority and status indicators that allow you to quickly
focus in on critical alarms
 Summary displays for different views of the alarm database based
on alarm status
 An historical list of alarms
 The capability to view subsets of alarms based on specific user-
defined criteria
 The capability to silence or temporarily mute workstation and
annunciator horns.
 Secured access to alarming functions dependent on user or system
responsibility
This set of resizable alarm displays providing a variety of current and
historic views of the process alarm database includes:
 A multi-page list of all the current alarms
 A single page of the most recent, active, unacknowledged alarms
with dynamically updating value and status fields
 Three summary displays specific to alarm status also with updating
values and statuses:
o all active, unacknowledged alarms
o all unacknowledged alarms that have returned to normal
o all active, acknowledged alarms
 A list of historized alarms related to the selected historian database
 An operations display for silencing horns, temporarily muting
horns, changing environments

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These displays allow you to respond to alarm conditions, filter and
analyze specific alarm data, and maintain alarm message files for
reporting purposes.
The Process or Alarm button in the Display Manager (DM) window
indicates the presence of alarms (both acknowledged and
unacknowledged) and provides access to Alarm Manager Displays.
Initially, the Current Alarm Display (CAD) appears and the other
displays are easily accessible from the CAD via its default Displays
menu:
 Most Recent Alarm display (MRA)
 New Alarm display (NEWALM)
 Unacknowledged Alarms display (UNACK)
 Acknowledged Alarms display (ACKALM)
 Alarm History display (AHD)
 Operations display (OPR)
These easy-to-use displays support the following features:
 A pre-configured number of alarms per screen or page
 Pre-configured alarm message information and formatting per
alarm type
 A status area for indication of current Alarm Manager and display
status, such as horns muted, match active, display paused, initial
call-up time
 Buttons for responding to alarm conditions, such as acknowledging
or clearing alarms, and for accessing additional alarm information
and process displays
 Pull-down menus for editing, viewing, and filing functions
 A pull-down menu for accessing other displays
 Pop-up menus for quick access to commonly used functions

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 A scroll bar and Go To Page option for moving easily through the
alarm list
Although a preconfigured set of alarm displays is provided, many aspects
of the displays and alarm message content are user configurable to
accommodate different process control applications and operational
needs. See the section on Alarm/Display Manager Configurator.

5.4 Historian

The Historian collects, stores, processes, and archives process data
from the control system to provide data for trends, Statistical Process
Control (SPC) charts, logs, reports, spreadsheets, and application
programs. The Historian software is an easy-to-use data collection tool
that allows the user to organize and enforce a plant data collection
philosophy. The Historian provides extensive data collection and
management functions, and data display functions for use by process
engineers or operators.
Typical historical data are process analog and/or digital variables
(points). The Historian can also collect and display application generated
messages. You can use the Historian to collect data in support of the
following production control functions:
 Cost accounting
 Equipment performance analysis
 Historical trending
 Information retrieval
 Inventory management
 Legal record maintenance
 Lost time analysis

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 Maintenance reporting
 Material accounting
 Process analysis
 Production reporting
 Quality control
The Historian can:
 Retrieve variables from process databases or accept data from
production control databases maintained by user application
programs.
 Perform built-in calculations on the collected data.
 Store calculated (reduced) data in a real time, relational database.
Application software in a plant-wide control system can access the
Historian database to obtain historical data for process control, production
control, and management information reporting.
You can use SPC chart displays of Historian data to monitor process
variables on-line via the Statistical Process Control Package (SPCP).
You can build displays for trending historical data via the Display Builder
and Display Configurator with Trending software.
Using the Report Writer, you can generate detailed reports of historical
data for management information.
Examples of Industrial Software that interface with the Historian are:
 Batch Plant Management
 Data Validator
 Display Manager
 Display Configurator with Trending
 Object Manager (for process data histories)
 Operator Action Journal
 Operator Message Interface
 Real-Time Data Base Manager

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 Spreadsheet
 Statistical Process Control Package
 System Monitor
 Report Writer

5.5 Draw

Figure 5.2 Draw
Draw is a display builder and configurator that allow you to create
and maintain dynamically updating process displays. Displays can
represent the plant, a process area or a detailed portion of the process.
You can draw basic objects using Draw's toolbars, menu items and
shortcut keys. You assign graphic attributes such as color and line style to
the objects, and then configure them to reflect process variable changes or
operator actions. Draw includes numerous palettes of objects such as
operator buttons, pumps, tanks, pipes, motors, valves and ISA symbols.
You can also create your own palettes for storing complex objects and
company-standard symbols. Displays can include faceplates, trends and
bitmapped images. You can easily edit your displays to reflect changes in
the process control scheme or to maximize operating efficiency and
security.

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5.5.1 Configuration

There are two ways of configuring a display object. You can:
1. Choose the Dynamic Update tab to connect one of the object's
attributes, such as visibility or fill level, to a process variable or a
file. With this type of configuration, changes in an attribute are
triggered dynamically by changes in the process variable. No
operator intervention is necessary.
2. Choose the Operator Action tab to connect the entire object to an
action, such as opening a display or executing a command. An
operator triggers the action by selecting the object.
An individual object can have both types of connections, although it can
have only one operator action.

5.5.2 Operator Actions

In a display configured for operator action, an operator can trigger
events by selecting an object (typically a button), moving a slider, or
typing text or a numeric value. In response to an operator action,
variables can be modified, a new display can open or an overlay can
appear.

While you can configure only one operator action for each display
object, you can trigger two or more events with a single operator action
by configuring an object with a View display command script.
Operator Actions include:
 Open Display
 Open Overlay
 Close Display/Overlay

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 Display Command
 Relative Pick
 Momentary Contact
 Ramp
 Connect Variable
 Move Horizontal or Vertical
 Numeric/Text Entry

5.5.3 Faceplates
A faceplate is a dynamic representation of control block
parameters. Draw provides a complete library of faceplates, ready to be
connected to any control block in the control database. In addition, you
can build your own faceplates using the standard Draw drawing tools.
To configure a faceplate, you need only define the Compound:block to
which the faceplate is connected. Draw automatically determines the
proper configuration attributes for the associated Compound:block.

5.5.4 Trends
Trend areas represent changing data values from the real-time
database and historian database. A data is displayed as a series of plotted
points connected by straight lines and scaled according to the high and
low limits configured for each trend line.

5.5.5 Group Displays
Group displays allow you to group faceplates and trends into
unique layouts to meet changing operational needs.

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5.6 View

Figure 5.3 View
View is a window into the system software, providing a user-
friendly interface to the total process. You can interact with any or all of
the real-time plant, field, and process data available in the system.
View provides:
 Direct access to dynamic process displays.
 Entry into user-configurable operating environments specific to
each user - the process engineer, process operator, and software
engineer.
 Execution of embedded real-time and historical trending.
 Service and display of process alarms via the Alarm Manager.
 An overview of the compounds and blocks in the control database
and access to block default detail displays via Select.
 Access to other applications, such as:
o Draw software for building and configuring dynamic user
graphics.
o System Management Displays for monitoring system
equipment health.
o Integrated Control Configurator for configuring the control
database.
o Historian for configuring the historization of data and system
messages.

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73
o Access to the four most recently used displays.
Additionally, with View you have:
 Flexibility in customizing environments to conform to your site
requirements.
 Rapid access to View while in other applications.
 Screen print utility.
 Window sizing options.
The multi-window capability of Solaris and Windows NT operating
systems allows you to monitor the information on a process control
display as well as access other applications without closing any window.

5.6.1 View Window

View Window contains the following features:
 A top menu bar for accessing displays, configurators, and other
applications as specified by the environment.
 A display bar of named display buttons or eight "thumbnail" mini-
display buttons for directly accessing process displays.
 A system bar with System and Process alarm buttons indicating
system and process health; a message bar with a dropdown list of
the latest messages; display of the current date and time.
 A status bar indicating the current display name, current operating
environment, Operator Action Journal logging name, printer
logging name, Historian name.

Using the control window menu, you can:
 Resize the View window automatically or manually.
 Move the window.

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5.6.2 Operating Environments

A collection of programs, utilities, and displays related to user
tasks is provided for each of the following: process operator, process
engineer, and software engineer. These environments, including menu
bars, menu content, and Display Bar content, can be modified to conform
to your site requirements. You can easily switch from one configured
environment to another. To secure environments against unauthorized
use, environment passwords can be configured and menu entries disabled
based on the environment.

5.7 Operator Action Journal

The Operator Action Journal is a record of specific operator actions
taken during process control operations. These actions generally consist
of manipulating certain Control Processor, and gateway parameters as
well as Application Processor, Application Workstation, and Workstation
Processor shared variables. Actions of this type are the ramping or direct
data entry of point values, toggling points, changing block statuses,
acknowledging block alarms, and horn muting. Operator action reporting
is limited to operator actions from the Display Manager, View, and
Alarm Manager. Also logged are environment change actions, scripts,
applics, and invoking other applications such as configuration.

When the Operator Action Journal feature is enabled, all operator
actions within the Display Manager, View, and the Alarm Manager that
change parameters in the process database are logged to a printer and/or
to the specified Historian database. These operator actions include
toggling points, ramping or direct data entry of new point values,

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75
changing block statuses, acknowledging block alarms, and other actions
such as horn muting.

Information logged as a result of each database change includes:
 Name of the Display Manager, FoxView, or Alarm Manager that
requested the database change.
 Compound:Block.Point for which the change was made.
 The "old value" TO "new value" text for non-packed Boolean.
 Current mask and data value for packed Boolean/long.
Following is an example of an Operator Action Journal Report.

Operator Action Journal Report

Tue Aug 1 1997 17:04:05 Page 1
08-02-97 07:57:08 GC3E31 SCRIPT /usr/fox/hi/init.cmds
08-02-97 07:57:15 GC3E31 ChgEnv Init_Env ->Init_env
08-02-97 07:58:19 GC3E31 ChgEnv Init+Env ->Proc_Eng_Env
08-02-97 08:00:34 CG3E31 UC01_LEAD :SINE .OUT 16.18 to 46.18
08-02-97 08:00:54 GC3E31 UC01_LEAD :SINE .MA Manual to Auto
08-02-97 08:00:57 GC3E31 UC01_LEAD :SINE .LR Remote to Local
08-02-97 08:01:01 GC3E31 UC01_LEAD :SINE .MA Auto to Manual

5.8 Control Configuration

Process control for DCS is based on the concepts of compounds
and blocks. A compound is a logical collection of blocks that performs a
control strategy. A block is a member of a set of algorithms that performs
a certain control task within the compound structure. Figure 7.4 shows
the compound/block relationship.
The compound provides the basis for the integration of:
 Continuous control
 Ladder logic
 Sequential control.

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76
Within this structure, any block in any compound can be connected to
any other block in any other compound in the system. The entire
compound structure can be viewed through the workstation display.
The block contains parameters that have values of the types: Real,
Boolean, Packed Boolean, Boolean Long, Integer, or String.

Figure 5.4 Compound/Blocks relationship

5.8.1 Compound Functions

The compound supports the following functions for the related blocks:
 Process alarm priority, alarm inhibiting, and alarm grouping
 Sequence status notification (see Sequential Control section)
 Phasing for execution load leveling at execution time.

5.8.2 Compound/Block Process Alarming

Alarms and status messages are generated by specific alarm blocks
and by alarm options in selected blocks. Alarms have five levels of
priority, 1-5, (where 1 = highest priority) that enable you to quickly focus
on the most important plant alarm conditions. An alarm priority of 0
indicates the absence of any alarm. These are summarized in a single
alarm summary parameter for each compound. This parameter contains
the priority of the highest current alarm in that compound. To reduce

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nuisance alarms, alarms can be inhibited at the compound level on a
priority level basis. Alarms can also be inhibited at the block level, on
either an alarm type basis, or an overall basis.
Alarms are initiated by the blocks within the compound. Alarm
messages are then sent to groups of stations or applications (for example,
Workstations, Historians, Printers) according to configured alarm groups.
The UNACK alarm acknowledge output parameter allows the user to
propagate alarm acknowledge actions to all blocks in a compound.
Stations, applications, and devices corresponding to various alarm
destination groups are configured at the compound level or at the station
level in the case of station compounds.
Group numbers for individual block alarm types are configured at the
block level.
5.8.3 Compound/Block Phasing
A user-defined phase number can be assigned to each compound
using a range of integer values that varies with assigned period. Phasing
allows the starting time of one compound/block to lead or lag the starting
time of another compound/block, thereby leveling the block processor
load.

5.8.4 Compound Attributes
The compound has the following attributes:
 Name: User-defined name that must be system-unique and no more
than 12 characters in length. The name can be any mix of numeric
(0 to 9), upper case alphabetic (A to Z), and the underscore (_).
 Descriptor: 32-character field for user-defined identification.
 On/Off: Parameter that enables or disables the execution of all
blocks within the compound, where: 1 = on; 0 = off.

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5.8.5 Compound/Block Parameters

Compound and block parameters contain values that are of one of
the types Real, String, Integer, Short Integer, Long Integer, Boolean,
Packed Boolean, Packed Long, or Character. Additionally, parameters are
defined as being configurable, and either connectable/settable, not
connectable/not settable, or a combination that is dependent upon the
compound, block, and state.

5.8.5.1 Configurable Parameters

Configurable parameters are those parameters that can be defined
through the Integrated Control Configurator. They can be displayable
only, or displayable and editable.

5.8.5.2 Connectable Parameters

Connectable parameters are those parameters of the user interface
in which secured, change-driven connections may be made between
network stations, or as local direct connections within the same station.
Each connection consists of a connectable source and a connectable sink.
Output parameters (all outputs are connectable) are sources, while a
connectable input may be a sink or a source, or both.
Certain parameters that may be considered functional inputs (such as SPT
in the PID blocks, and RATIO in the RATIO block) are settable but not
connectable. A connectable parameter has a value record that contains the
parameter's value, its status, and its designated value type (Real, Boolean,
or Integer).

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5.8.5.3 Input Parameters

Input parameters are connectable types that are the receivers of
data from other connectable parameters via a path connection.
If no source path is specified during configuration, then the resident data
of the value record is the actual "source" of data. It can be either the
initial default or configured value, or a new value through a SET call to
the input parameter.
If a source path is specified, then the data value is an output parameter of
the same or another block, or a shared variable, thereby securing the
input. By linking a shared variable to a block input during configuration,
the user can establish a long-term secured connection between a remote
application program and the block input.

5.8.5.4 Output Parameters

All output parameters are connectable data sources that have value
records. There are two types: settable and nonsettable. The settability of a
settable output is controlled by the secured status of the value record. The
secured status is dependent on whether the block's operational mode is in
Auto or in Manual. In either Auto or Manual, nonsettable output
parameters cannot be written by any other source under any conditions.

Settable outputs may be conditionally released by the block
algorithm in the Manual mode. In Manual, the block unsecures settable
output parameters. They can then be written by other tasks via SET calls.
When the block switches to Auto, the block secures and updates its
output parameter(s).

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5.8.5.5 Nonconnectable Parameters

Nonconnectable parameters have no value records and are not
linkable. They mainly consist of string-type variables like NAME, or
nonsettable parameters that are used in the configurator only, for
example, block options. Local algorithm variables are also
nonconnectable. Nonconnectable parameters are generally accessible
through GET calls.
There is also a class of nonconnectable input parameters that comprise the
block user interface which can be manipulated through SET calls. An
example is an alarm deadband.

5.9 Role Play
Each trainee should introduce one of the main applications:
8. System Management.
9. Historian
10. Graphics Applications.
11. Control Configurator.
12. Operator Journal
13. Alarm System.

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Chapter 6
Installation

6.1 Learning objectives

 To be able to define installation procedure for each component.

6.2 Modular Industrial Console

The Modular Industrial Console (MIC) provides flexible mounting
arrangements for components. The MIC can incorporate a mixture of
equipment: console displays, input devices, processors, Fieldbus
modules, data storage devices, and so on.
Modular Industrial Consoles support powerful multiple-screen, real-time
display software interactions. This hardware/software combination allows
console resources to be allocated with the flexibility to meet changing
day-to-day needs. Multi-screen consoles enable comprehensive handling
of more plant information in a coordinated fashion.
The MIC product line (Figure 6.1) allows a highly flexible
packaging configuration of console equipment. Individual MIC modules
are joined on-site to provide a customized configuration using standard
components.
This modular approach offers you combinations of single-screen and
multi-screen real-time display software interactions as required at a given
console. There are, however, specific allocations for mounting equipment
within configurations.

Distributed Control System and Programmable Logic Control
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Figure 6.1 MIC Arrangement
The MIC is built up from four basic pieces of equipment, each of which
is individually configurable:
 MIC bay - basic full bay unit, full height, 27-inches wide, with bay
module
 Spacer module - storage space between MIC full bay units
 Desktop/printer bay - a rear bay similar to the full bay unit's with a
flat tabletop
 Free standing table - a basic multipurpose table.

6.3 System Equipment
6.3.1 Unloading
The system units must be designed to withstand vibration and
shock normally encountered during shipping and installation; however,
extreme shocks and vibration should be avoided. The system units may
be moved from the transportation vehicle to their intended locations by
forklift or manual jack truck. If practical, all major movements of the
units should be accomplished before the units are unpacked.

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6.3.2 Unpacking Procedure

The following unpacking procedure applies, in general, to all system
units:
 Inspect the exterior of the shipping carton for obvious damage. (Any
noticeable damage should be indicated in the shipper's bill of lading.)
 Verify that the equipment received is that described in the bill of
lading.
 Remove shipping straps, shipping shroud, and other packing materials,
such as polyethylene bags and Styrofoam cushioning materials.
 If the unit is attached to a skid, remove all shipping hardware and
hold-down bolts used to fasten the unit to the skid. Separate the skid
from the unit.
 Ensure that the appropriate interconnecting cables are present, by
comparing the cable part numbers and quantities with those listed in
the bill of lading.

6.3.3 System Power Checks

Perform the following checks before you install the equipment:
 Check that all the required ac or dc power distribution network lines
are installed.
 Check that the appropriate number of ac power outlets are installed
and spaced appropriately.
 Switch on main system power.
 Using a multimeter, check that the appropriate operating voltage exists
at each ac outlet or connection point.
 Switch off main system power.

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6.3.4 Industrial Enclosures Mounting Procedures
Figure 6.2 shows a single dual-height modular mounting structure
area for containing processors and modules in an Industrial Enclosure.

Figure 6.2 Industrial Enclosure Mounting Structure Area
Enclosures are designed for floor mounting, and accept processor
modules, Fieldbus modules, and data storage devices. Wires, cables, and
conduits can enter either the bottom or the top of the enclosure. Side
doors provide access to the wiring areas. Additionally, the doors can be
mounted to open from left-to-right or right-to-left.
Industrial Enclosures are available in two configurations, vented
and sealed. The vented configuration has openings at the top and bottom
to provide ventilation, and has a metal plate, with gasket, at the bottom
for electrical protection purposes. A sealed enclosure has metal plates,
with gaskets, at the top and bottom to provide a watertight seal.
1. Check that mounting holes have been drilled in floor. If they have
not, proceed as follows. (If below-floor cabling is to be employed,
refer to the Site Planning document for information on the
recommended size and placement of the floor cutout.)
a. Place enclosure in desired location.
b. Mark hole locations.
c. Move the enclosure away from the markings.
d. Drill holes in floor.

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2. If the enclosure is the vented type and conduit entry is to be from
the bottom:
a. Drill or punch the bottom conduit enclosure plate, and
provide appropriate conduit fittings.
b. Place the conduit enclosure plate on the floor, in the precise
location that the enclosure is to be mounted.
c. Go to Step 6.
3. If the enclosure is the vented type, and conduit entry is to be from
the top:
a. Remove the vent cap and top conduit enclosure plate(s).
b. Drill or punch the conduit enclosure plate(s).
c. Replace the vent cap and conduit enclosure plate(s).
d. Place the enclosure plate on the floor, in the precise location
that the enclosure is to be mounted.
e. Go to Step 6.
4. If the enclosure is the sealed type and conduit entry is to be from
the bottom:
a. Drill or punch the bottom conduit enclosure plate, and
provide appropriate conduit fittings for a watertight seal.
b. Place the conduit enclosure plate on the floor, in the precise
location that the enclosure will be mounted.
c. Go to Step 6.
5. If the enclosure is the sealed type and conduit entry is to be from
the top:
a. Remove the top conduit enclosure plate.
b. Drill or punch the conduit enclosure plate, and provide
appropriate conduit fittings for a watertight seal.
c. Replace the conduit enclosure plate.

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6. Position the enclosure, with mounting gasket and enclosure plate,
so that the holes in the enclosure base, gasket, and enclosure plate
are aligned with the mounting holes in the floor.
7. Install two bolts, with flat washers and lockwashers, in diagonally
opposite mounting holes. (Do not tighten.)
8. Install two more bolts, with flat washers and lockwashers, in the
other two diagonally opposite mounting holes. (Do not tighten.)
9. Install the remaining bolts, with flat washers and lockwashers, in
the center mounting holes.
10. Tighten all bolts evenly and equally, working from center to
outside bolts, being careful not to overtighten. Maximum torque
should be applied carefully.

6.4 Software Installation

The Installation Phase performs the installation of software
packages. Installation of software packages is performed by vendor
representative on target stations.

6.5 Discussion

Initiate a dialogue between trainees to discuss their own
experiences and notes about different installation phases related to the
text in this chapter.

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Chapter 7
Maintenance

7.1 Learning objectives

 Understand maintenance philosophy and procedures.

7.2 Maintenance Philosophy

The maintenance approach is oriented toward module replacement.
The use of diagnostics, fault location tables, and troubleshooting guides
described in system document, as well as the presence of status lamps
(LEDs) on each module, enables isolation of problems to the module
level. In addition, any module can be replaced without affecting the
operation of any other module, including the module of a fault-tolerant
pair.

7.3 Preventive Maintenance

The design of DCS equipment and associated peripheral devices is
such that scheduled preventive maintenance on the equipment is limited
to visual inspections, periodic cleaning procedures, and adjustment of
system modules if necessary. While performing these routines, you
should check for damaged cables, loose connections, inoperative fans and
indicator lamps, wear or binding of drives and fan motors, and take
appropriate corrective action.

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7.3.1 Enclosures
Perform a general visual inspection and exterior cleaning of each
enclosure after the first six months of service. Approximately every
12 months thereafter perform the same, depending on local environmental
conditions. Preventive maintenance procedures for enclosures include the
following:
1. Wipe down the exterior of the enclosure with a soft cloth. A damp
cloth and/or a nonabrasive cleaner can be used for hard-to-remove
spots.
2. Clean any dust buildup from module heat fins. Use a soft cloth. If
heat fins are accessible from rear of enclosure, they can be cleaned
during normal operation. Otherwise, modules can be removed and
cleaned from front of enclosure during routine equipment
shutdowns.
3. Check fans (if installed) for proper operation.
4. Check module status indicators for proper operation.
Green light indicates normal operation.
Red light indicates faulty operation.

7.3.2 Enclosures Air Filters
The vented configurations of all metal enclosures have an air filter
located inside the door, behind the vents. Periodically check the condition
of the filter for dust/dirt accumulation. Perform the following steps to
check the condition of the filter:
1. Locate the plastic assembly that retains the filter that is on the
inside of the door behind the vents.
2. Unsnap the plastic assembly from the vents and remove the filter.
3. Wash and replace the filter, or if desired, install a new filter, and
snap the filter retainer assembly back onto the vent assembly.

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7.3.3 Modular Industrial Workstations
Perform a general visual inspection and exterior cleaning of each
workstation as often as necessary to ensure proper operation of the
equipment. Preventive maintenance procedures for the workstations
should include the following:
1. Wipe down the exterior of the enclosure with a soft cloth. A damp
cloth and/or a nonabrasive cleaner can be used for hard-to-remove
spots.
2. Clean any dust buildup on disk drives (especially the signal
connection areas), keyboards, control panels, and monitors. Use a
soft cloth.
3. Check fans (if installed) for proper operation.
4. Check module status indicators for proper operation.
Green light indicates normal operation.
Red light indicates faulty operation.

7.3.4 Monitor-Based Peripheral Devices
As a rule, preventive maintenance on these devices should be
limited to cleaning only and should be performed as often as necessary, or
at least every twelve months.
Wipe down the exterior of the device (excluding the monitor) with a soft
cloth. A damp cloth and/or nonabrasive cleaner can be used for hard-to-
remove spots.
To clean the monitor, proceed as follows:
1. Select a screen that does not have direct access to the process, for
example, the Initial display.
2. Remove power from the GCIO unit (annunciators are also
deactivated).

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3. Turn the monitor's power off. Do not move the mouse or depress
any keys while the monitor is off.
4. Dampen - do not saturate - a clean, lint-free cloth with liquid
glass cleaner.
5. Clean the screen by wiping with damp cloth, using circular wiping
motion to avoid streaks.
6. Carefully dry the screen by wiping with a second clean, lint-free
cloth.
7. Restore power to the monitor and GCIO.

7.3.5 Printers
All printers should be serviced every six months (or after 300 hours
of operation), whichever occurs first. Refer to the associated printer user's
guide (packed with the printer) and perform the following:
1. Perform a general visual inspection and cleaning of the printer.
2. Remove printer cover and inspect internal moving parts for signs of
wear, broken or loose parts, frayed cables, and so on.
3. Take a clean, dry, soft cloth and dust the area around carriage shaft
and platen. Remove any loose particles of paper and dust.
4. Lubricate printer as described in associated service instructions.
5. Restore printer power.

7.3.6 Keyboard
A keyboard should be cleaned at a frequency determined by the
environment in which it is used.
1. Use a soft, lint-free cloth dampened with a mild detergent solution
to clean the keys and large surfaces.
2. Clean confined areas between the keys with a vacuum cleaner
equipped with a fine brush attachment.

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7.3.7 Mouse
The following care and cleaning procedure applies to both the inner
and outer area of the mouse:
1. The mouse is a very precise mechanical device, so handle it with
care. Do not drop, hit, or otherwise subject it to shock.
2. Do not pull on the cable. It may cause damage to both the cable
and connector.
3. Do not carry the mouse by holding onto the cable.
4. Be sure to place a clean sheet of paper or use a mouse pad between
the mouse and the flat surface. Dirt and grit could collect on the
ball. Try not to touch the ball on the bottom.
5. Do not use the mouse in extreme temperatures or in direct sunlight.
6. Do not allow the mouse to come in contact with liquid spills
(water, solutions, and so forth).
7. The mouse housing should be cleaned with a lint-free cloth using a
mild detergent. Use an unsoiled lint-free cloth to dry housing.
8. Do not disassemble the mouse. If the ball in the unit needs to be
cleaned, remove it from the lower case by detaching the cover to
the housing. Do not remove all the screws to remove the ball.
9. Use a lint-free cloth with mild detergent to clean the ball, and an
unsoiled cloth to dry it.

7.3.8 Data Storage Devices
1. Blow away any lint or dust accumulation on or near the face of the
floppy disk and streaming tape drive casings.
2. Clean the outer plastic surface of the drive with a lint-free cloth or
a sponge slightly dampened with water. Wipe off residue and dry
with soft, lint-free cloth. Do not use abrasive cleaners, solvents, or
strong detergents.

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3. Blow away any lint or dust accumulation on the signal and power
connectors at the rear of the drive.
4. For the streaming tape drive, clean the head using only Freon TF
and polyurethane swabs, commonly available with VCR head
cleaning kits. Wet the swab with the Freon TF solution, and wipe
the head using an up and down motion. Use a dry swab to clean
any remaining residue from the head.

7.4 Fault Analysis

Through the System Management facility, you can monitor the
health of the system and perform diagnostic tests on all the system
stations and associated peripheral devices.

7.4.1 Startup Diagnostics
Startup diagnostics are invoked automatically as a result of a
power-on reset, an error, or an off-line diagnostic command. The
diagnostics exist in each station at all times and are of two basic types:
 Reportable diagnostic - Tests a station function which, if faulty,
does not prevent the error from being reported over the network.
 Nonreportable diagnostic - Tests a station function which, if faulty,
inhibits the station from communicating over the network.

7.4.2 On-line Diagnostics
On-line diagnostics consist of Carrierband LAN LI (LAN
Interface) Cable Tests and Nodebus Cable Tests. These tests are either
operator-initiated or automatically invoked to isolate faults and to check
the integrity of the communication path.

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7.4.3 Off-line Diagnostics
Off-line diagnostics are used to check for, or verify the proper
"independent" operation of a station's internal components. These tests do
not verify any external reason for failure, thus they can be individually
bench tested without regard to the station's subsystem configuration.

7.5 Corrective Maintenance
7.5.1 Module Status Indicators
All power modules, Processor modules, LAN modules, and
Fieldbus Modules have red and green status indicators that operate in
accordance with the maintenance manual codes.

7.5.2 I/A Series Module Replacement
The maintenance approach is oriented toward module replacement.
Fault analysis provides assistance with isolating station and peripheral
faults. The presence of status lamps (LEDs) on each module enables an
initial detection of problems that can exist on the module level. In
addition, any module can be replaced without affecting the operation of
any other module, including the other module of a fault-tolerant pair.
Replacement of modules is similar to installation, which is described in
the System Equipment Installation.

7.6 Discussion

Exchange of ideas with trainees to talk about their own experiences
and comments about maintenance related to the text in this chapter.

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Chapter 8
Power Distribution

8.1 Learning objectives

 Understand power distribution of control systems.

8.2 Power Connections

Main power consists of primary and secondary power. Note the
voltage and main power distribution requirements for each enclosure
before you connect main power. The power should be connected through
an uninterruptible power supply.

8.3 Connection Procedure

To connect the power lines proceed as follows:
1. Switch off main system power.
2. Open the right side door of the enclosure to access the junction
boxes. (Two junction boxes are located in the field termination
area.)
3. Place the junction box power switches in the OFF position.
4. Remove the bottom cover from each junction box.
5. Route the power lines to the junction boxes.
6. Connect the power lines.
7. Replace the junction box covers.
8. Switch ON the main system power.

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8.4 Earth Connections

To make earth connections to the metal enclosures, locate one of
the tapped holes along the bottom interior of the enclosure (see Figure
8.1). Use a ring type solderless crimp connector appropriate for the size
of wire used, and use a star-type lock washer between the connector and
the enclosure chassis.

Figure 8.1 Metal Enclosures, Earth Connection

8.5 Discussion

Discuss power distribution schemes.

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Chapter 9
PLC Fundamentals

9.1 Learning objectives

• Know general PLC issues
• Understand the operation of a PLC
• Understand the different types of inputs and outputs.

9.2 Introduction

Control engineering has evolved over time. In the past humans
were the main methods for controlling a system. More recently electricity
has been used for control and early electrical control was based on relays.
These relays allow power to be switched on and off without a mechanical
switch. It is common to use relays to make simple logical control
decisions. The development of low cost computer has brought the most
recent revolution, the Programmable Logic Controller (PLC). The advent
of the PLC began in the 1970s, and has become the most common choice
for manufacturing controls. PLCs have been gaining popularity on the
factory floor and will probably remain predominant for some time to
come. Most of this is because of the advantages they offer.
• Cost effective for controlling complex systems.
• Flexible and can be reapplied to control other systems quickly
and easily.
• Computational abilities allow more sophisticated control.
• Trouble shooting aids make programming easier and reduce
downtime.

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• Reliable components make these likely to operate for years before
failure.

9.3 Hardware

Many PLC configurations are available, even from a single vendor.
But, in each of these there are common components and concepts. The
most essential components are:
 Power Supply - This can be built into the PLC or be an external
unit. Common voltage levels required by the PLC (with and
without the power supply) are 24Vdc, 120Vac, 220Vac.
 CPU (Central Processing Unit) - This is a computer where ladder
logic is stored and processed.
 I/O (Input/Output) - A number of input/output terminals must be
provided so that the PLC can monitor the process and initiate
actions.
 Indicator lights - These indicate the status of the PLC including
power on, program running, and a fault. These are essential when
diagnosing problems.
The configuration of the PLC refers to the packaging of the components.
Typical configurations are listed below from largest to smallest as shown
in Figure 9.1.
 Rack - A rack is often large (up to 18” by 30” by 10”) and can hold
multiple cards. When necessary, multiple racks can be connected
together. These tend to be the highest cost, but also the most
flexible and easy to maintain.
 Mini - These are similar in function to PLC racks, but about half
the size.

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 Shoebox - A compact, all-in-one unit (about the size of a shoebox)
that has limited expansion capabilities. Lower cost, and
compactness make these ideal for small applications.
 Micro - These units can be as small as a deck of cards. They tend to
have fixed quantities of I/O and limited abilities, but costs will be
the lowest.
 Software - A software based PLC requires a computer with an
interface card, but allows the PLC to be connected to sensors and
other PLCs across a network.

Figure 9.1 Typical configuration of PLC

9.4 Inputs And Outputs

Inputs to, and outputs from, a PLC are necessary to monitor and
control a process. Both inputs and outputs can be categorized into two
basic types: logical or continuous. Consider the example of a light bulb. If
it can only be turned on or off, it is logical control. If the light can be
dimmed to different levels, it is continuous. Continuous values seem
more intuitive, but logical values are preferred because they allow more
certainty, and simplify control. As a result most controls applications (and
PLCs) use logical inputs and outputs for most applications. Hence, we
will discuss logical I/O and leave continuous I/O for later.

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Outputs to actuators allow a PLC to cause something to happen in a
process. A short list of popular actuators is given below in order of
relative popularity.
 Solenoid Valves - logical outputs that can switch a hydraulic or
pneumatic flow.
 Lights - logical outputs that can often be powered directly from
PLC output boards.
 Motor Starters - motors often draw a large amount of current when
started, so they require motor starters, which are basically large
relays.
 Servo Motors - a continuous output from the PLC can command a
variable speed or position.
Outputs from PLCs are often relays, but they can also be solid state
electronics such as transistors for DC outputs or Triacs for AC outputs.
Continuous outputs require special output cards with digital to analog
converters.
Inputs come from sensors that translate physical phenomena into
electrical signals. Typical examples of sensors are listed below in relative
order of popularity.
 Proximity Switches - use inductance, capacitance or light to detect
an object logically.
 Switches - mechanical mechanisms will open or close electrical
contacts for a logical signal.
 Potentiometer - measures angular positions continuously, using
resistance.
 LVDT (linear variable differential transformer) - measures linear
displacement continuously using magnetic coupling.

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Inputs for a PLC come in a few basic varieties, the simplest are AC
and DC inputs. Sourcing and sinking inputs are also popular. This output
method dictates that a device does not supply any power. Instead, the
device only switches current on or off, like a simple switch.
 Sinking - When active the output allows current to flow to a
common ground. This is best selected when different voltages are
supplied.
 Sourcing - When active, current flows from a supply, through the
output device and to ground. This method is best used when all
devices use a single supply voltage.
This is also referred to as NPN (sinking) and PNP (sourcing). PNP is
more popular.

9.5 Operation Sequence

All PLCs have four basic stages of operations that are repeated
many times per second. Initially when turned on the first time it will
check its own hardware and software for faults. If there are no problems it
will copy all the input and copy their values into memory, this is called
the input scan. Using only the memory copy of the inputs the ladder logic
program will be solved once, this is called the logic scan. While solving
the ladder logic the output values are only changed in temporary memory.
When the ladder scan is done the outputs will updated using the
temporary values in memory, this is called the output scan. The PLC now
restarts the process by starting a self check for faults. This process
typically repeats 10 to 100 times per second as is shown in Figure 9.2.

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Figure 9.2 PLC Scan
 Self test - Checks to see if all cards error free, reset watch-dog
timer, etc. (A watchdog timer will cause an error, and shut down
the PLC if not reset within a short period of time - this would
indicate that the ladder logic is not being scanned normally).
 Input scan - Reads input values from the chips in the input cards,
and copies their values to memory. This makes the PLC operation
faster, and avoids cases where an input changes from the start to
the end of the program (e.g., an emergency stop). There are special
PLC functions that read the inputs directly, and avoid the input
tables.
 Logic solve/scan - Based on the input table in memory, the
program is executed 1 step at a time, and outputs are updated. This
is the focus of the later sections.
 Output scan - The output table is copied from memory to the
output chips. These chips then drive the output devices.

The input and output scans often confuse the beginner, but they are
important. The input scan takes a snapshot of the inputs, and solves the
logic. This prevents potential problems that might occur if an input that is
used in multiple places in the ladder logic program changed while half
ways through a ladder scan and thus changing the behaviors of half of the
ladder logic program. This problem could have severe effects on complex
programs. One side effect of the input scan is that if a change in input is
too short in duration, it might fall between input scans and be missed.

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When the PLC is initially turned on the normal outputs will be turned off.
This does not affect the values of the inputs.

9.5.1 The Input and Output Scans
When the inputs to the PLC are scanned the physical input values
are copied into memory. When the outputs to a PLC are scanned they are
copied from memory to the physical outputs. When the ladder logic is
scanned it uses the values in memory, not the actual input or output
values. The primary reason for doing this is so that if a program uses an
input value in multiple places, a change in the input value will not
invalidate the logic. Also, if output bits were changed as each bit was
changed, instead of all at once at the end of the scan the PLC would
operate much slower

9.5.2 The Logic Scan
Ladder logic programs are modeled after relay logic. In relay logic
each element in the ladder will switch as quickly as possible. But in a
program elements can only be examines one at a time in a fixed sequence.
The ladder logic will be interpreted left-to-right, top-to-bottom. The
ladder logic scan begins at the top rung. At the end of the rung it
interprets the top output first, and then the output branched below it. On
the second rung it solves branches, before moving along the ladder logic
rung.

9.5.3 PLC Status
The lack of keyboard and other input-output devices is very
noticeable on a PLC. On the front of the PLC there are normally limited
status lights. Common lights indicate;
 Power on - this will be on whenever the PLC has power.

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 Program running - this will often indicate if a program is running,
or if no program is running.
 Fault - this will indicate when the PLC has experienced a major
hardware or software problem.
These lights are normally used for debugging. Limited buttons will also
be provided for PLC hardware. The most common will be a run/program
switch that will be switched to program when maintenance is being
conducted, and back to run when in production. This switch normally
requires a key to keep unauthorized personnel from altering the PLC
program or stopping execution. A PLC will almost never have an on-off
switch or reset button on the front. This needs to be designed into the
remainder of the system.

9.6 Role Play

Conduct role plays for:
1. Introduce PLC and benefits.
2. Describe PLC hardware.
3. Introduce various inputs and outputs.
4. Describe PLC scan sequence.

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Chapter 10
Ladder Logic and SFC

10.1 Learning objectives

• To be able to write simple ladder logic programs
• Understand basic functions for calculations and comparisons.
• Be able to develop SFCs, sequential flow charts, for a process.

10.2 Ladder Logic

Ladder logic is the main programming method used for PLCs. As
mentioned before, ladder logic has been developed to mimic relay logic.
Relays are used to let one power source close a switch for another (often
high current) power source, while keeping them isolated. An example of a
relay in a simple control application is shown in Figure 12.1. In this
system the first relay on the left is used as normally closed, and will allow
current to flow until a voltage is applied to the input A. The second relay
is normally open and will not allow current to flow until a voltage is
applied to the input B. If current is flowing through the first two relays
then current will flow through the coil in the third relay, and close the
switch for output C. This circuit would normally be drawn in the ladder
logic form. This can be read logically as C will be on if A is off and B is
on.

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Figure 10.1 Simple Relay Control.
The example in Figure 10.1 does not show the entire control
system, but only the logic. When we consider a PLC there are inputs,
outputs, and the logic. Figure 10.2 shows a more complete representation
of the PLC. Here there are two inputs from push buttons. We can imagine
the inputs as activating 24V DC relay coils in the PLC. This in turn drives
an output relay that switches 115V AC, which will turn on a light. Note,
in actual PLCs inputs are never relays, but outputs are often relays. The
ladder logic in the PLC is actually a computer program that the user can
enter and change. Notice that both of the input push buttons are normally
open, but the ladder logic inside the PLC has one normally open contact,
and one normally closed contact. Do not think that the ladder logic in the
PLC needs to match the inputs or outputs. Many beginners will get
caught trying to make the ladder logic match the input types.

Figure 10.2 PLC with Relays.

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Many relays also have multiple outputs (throws) and this allows an
output relay to also be an input simultaneously. The circuit shown in
Figure 10.3 is an example of this; it is called a seal in circuit or latch
circuit. In this circuit the current can flow through either branch of the
circuit, through the contacts labeled A or B. The input B will only be on
when the output B is on. If B is off, and A is energized, then B will turn
on. If B turns on then the input B will turn on and keep output B on even
if input A goes off. After B is turned on the output B will not turn off.

Figure 10.3 Latch circuit

10.2.1 Ladder Logic Inputs

PLC inputs are easily represented in ladder logic. Below there are
two types of inputs shown, normally open and normally closed inputs.

10.2.2 Ladder Logic Outputs

In ladder logic there are multiple types of outputs, but these are not
consistently available on all PLCs. Some of the outputs will be externally
connected to devices outside the PLC, but it is also possible to use

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internal memory locations in the PLC. Five types of outputs are shown
below. The first is a normal output, when energized the output will turn
on, and energize an output. The circle with a diagonal line through is a
normally on output, when energized the output will turn off. This type of
output is not available on all PLC types. When initially energized the
OSR (One Shot Relay) instruction will turn on for one scan, but then be
off for all scans after, until it is turned off. The L (latch) and U (unlatch)
instructions can be used to lock outputs on. When an L output is
energized the output will turn on indefinitely, even when the output coil
is deenergized. The output can only be turned off using a U output.

10.2.3 Programming

The first PLCs were programmed with a technique that was based
on relay logic wiring schematics. This eliminated the need to teach the
electricians, technicians and engineers how to program a computer - but,
this method has stuck and it is the most common technique for
programming PLCs today. An example of ladder logic can be seen in
Figure 10.4. To interpret this diagram, imagine that the power is on the

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vertical line on the left hand side, we call this the hot rail. On the right
hand side is the neutral rail. In the figure there are two rungs, and on each
rung there are combinations of inputs (two vertical lines) and outputs
(circles). If the inputs are opened or closed in the right combination the
power can flow from the hot rail, through the inputs, to power the
outputs, and finally to the neutral rail. An input can come from a sensor,
switch, or any other type of sensor. An output will be some device
outside the PLC that is switched on or off, such as lights or motors. In the
top rung the contacts are normally open and normally closed. This means
if input A is on and input B is off, then power will flow through the
output and activate it. Any other combination of input values will result
in the output X being off.

Figure 10.4 Simple Ladder Logic Diagram
The second rung of Figure 10.4 is more complex, there are actually
multiple combinations of inputs that will result in the output Y turning on.
On the left most part of the rung, power could flow through the top if C is
off and D is on. Power could also (and simultaneously) flow through the
bottom if both E and F are true. This would get power half way across the
rung, and then if G or H is true the power will be delivered to output Y.

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10.2.4 Move Functions
The simple MOV will take a value from one location in memory
and place it in another memory location. Examples of the basic MOV are
given in Figure 10.5. When A is true the MOV function moves a floating
point number from the source to the destination address.

Figure 10.5 MOV function

10.2.5 Mathematical Functions
Mathematical functions will retrieve one or more values, perform
an operation and store the result in memory. Figure 10.6 shows an ADD
function that will retrieve values from N7:4 and F8:35, convert them both
to the type of the destination address, add the floating point numbers, and
store the result in F8:36. The function has two sources labelled source A
and source B.

Figure 10.6 Mathematical Functions

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10.2.6 Block Operations
A basic block function is shown in Figure 10.7. This COP (copy)
function will copy an array of 10 values starting at N7:50 to N7:40.

Figure 10.7 Copy Function
10.2.7 Comparison of Values
Comparison functions are shown in Figure 10.8. Previous function
blocks were outputs, these replace input contacts. The example shows an
EQU (equal) function that compares two floating point numbers. If the
numbers are equal, the output bit B3:5/1 is true, otherwise it is false.

Figure 10.8 Comparison Functions
10.2.8 Boolean Functions

Figure 10.9 shows Boolean algebra functions. The function shown
will obtain data words from bit memory, perform an AND operation, and
store the results in a new location in bit memory. These functions are all
oriented to word level operations. The ability to perform Boolean
operations allows logical operations on more than a single bit.

Figure 10.9 Boolean Functions

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10.3 Sequential Flow Charts

Sequential Function Charts (SFCs) have been developed to
accommodate the programming of more advanced systems. These are
similar to flowcharts, but much more powerful. The example seen in
Figure 10.10 is doing two different things.

To read the chart, start at the top where is says start. Below this
there is the double horizontal line that says follow both paths. As a result
the PLC will start to follow the branch on the left and right hand sides
separately and simultaneously. On the left there are two functions the first
one is the power up function. This function will run until it decides it is
done, and the power down function will come after. On the right hand
side is the flash function; this will run until it is done.

These functions look unexplained, but each function, such as
power up will be a small ladder logic program. This method is much
different from flowcharts because it does not have to follow a single path
through the flowchart.

Figure 10.10 SFC Simple example

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The basic elements of an SFC diagram are shown in Figure 10.11.

Figure 10.11 Basic Elements of SFC

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A simple SFC for controlling a stamping press is shown in Figure
10.12. (Note: this controller only has a single thread of execution, so it
could also be implemented with state diagrams, flowcharts, or other
methods.) In the diagram the press starts in an idle state. When an
automatic button is pushed the press will turn on the press power and
lights. When a part is detected the press ram will advance down to the
bottom limit switch. The press will then retract the ram until the top limit
switch is contacted, and the ram will be stopped. A stop button can stop
the press only when it is advancing. (Note: normal designs require that
stops work all the time.) When the press is stopped a reset button must be
pushed before the automatic button can be pushed again. After step 6 the
press will wait until the part is not present before waiting for the next
part. Without this logic the press would cycle continuously.

Figure 10.12 SFC for Controlling a Stamping Press

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10.4 Case Study

Each Trainee should try to develop the following:
1. Ladder Logic for pump operation connected to the suction of a tank
where two level switches are available for automatic operation and
two push buttons are for start and stop.
2. SFC for loading three tanks through different valve. Tank 1 is load
first, and then tanks 2 and three are loaded simultaneously. If the
pressure switch on pump discharge line is alarming then tank 2
stops loading from pump and tank 1 would transfer to tank through
different line. Tank 3 continues to load from pump.

.

PSL
T1 T2 T3

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Appendix A
Electrical Relay Diagram
And
P&ID Symbols

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Appendix B
Serial Communication

B.1 Introduction

Multiple control systems will be used for complex processes. These
control systems may be PLCs, but other controllers include robots, data
terminals and computers. For these controllers to work together, they
must communicate. This chapter will discuss communication techniques
between computers, and how these apply to PLCs. The simplest form of
communication is a direct connection between two computers. A network
will simultaneously connect a large number of computers on a network.
Data can be transmitted one bit at a time in series, this is called serial
communication. Data bits can also be sent in parallel. The transmission
rate will often be limited to some maximum value, from a few bits per
second, to billions of bits per second. The communications often have
limited distances, from a few feet to thousands of miles/kilometers.
Data communications have evolved from the 1800’s when
telegraph machines were used to transmit simple messages using Morse
code. This process was automated with teletype machines that allowed a
user to type a message at one terminal, and the results would be printed
on a remote terminal. Meanwhile, the telephone system began to emerge
as a large network for interconnecting users. In the late 1950s Bell
Telephone introduced data communication networks, and Texaco began
to use remote monitoring and control to automate a polymerization plant.
By the 1960s data communications and the phone system were being
used together. In the late 1960s and 1970s modern data communications
techniques were developed. This included the early version of the

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Internet, called ARPAnet. Before the 1980s the most common computer
configuration was a centralized mainframe computer with remote data
terminals, connected with serial data line.
In the 1980s the personal computer began to displace the central
computer. As a result, high speed networks are now displacing the
dedicated serial connections. Serial communications and networks are
both very important in modern control applications. An example of a
networked control system is shown in Figure B.1. The computer and PLC
are connected with an RS-232 (serial data) connection. This connection
can only connect two devices. Devicenet is used by the Computer to
communicate with various actuators and sensors. Devicenet can support
up to 63 actuators and sensors. The PLC inputs and outputs are connected
as normal to the process.

Figure B.1 Communication example

B.2 Serial Communication

Serial communications send a single bit at a time between
computers. This only requires a single communication channel, as
opposed to 8 channels to send a byte. With only one channel the costs are
lower, but the communication rates are slower. The communication

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channels are often wire based, but they may also be can be optical and
radio. Figure B.2 shows some of the standard electrical connections. RS-
232c is the most common standard that is based on a voltage change
levels. At the sending computer an input will either be true or false. The
line driver will convert a false value in to a Txd voltage between +3V to
+15V, true will be between -3V to -15V. A cable connects the Txd and
com on the sending computer to the Rxd and com inputs on the receiving
computer. The receiver converts the positive and negative voltages back
to logic voltage levels in the receiving computer. The cable length is
limited to 50 feet to reduce the effects of electrical noise. When RS-232 is
used on the factory floor, care is required to reduce the effects of
electrical noise - careful grounding and shielded cables are often used.

Figure B.2 Serial data standard

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The RS-422a cable uses a 20 mA current loop instead of voltage
levels. This makes the systems more immune to electrical noise, so the
cable can be up to 3000 feet long. The RS-423a standard uses a
differential voltage level across two lines, also making the system more
immune to electrical noise, thus allowing longer cables. To provide serial
communication in two directions these circuits must be connected in both
directions.

To transmit data, the sequence of bits follows a pattern, like that
shown in Figure B.3. The transmission starts at the left hand side. Each
bit will be true or false for a fixed period of time, determined by the
transmission speed.

A typical data byte looks like the one below. The voltage/current
on the line is made true or false. The width of the bits determines the
possible bits per second (bps). The value shown before is used to transmit
a single byte. Between bytes, and when the line is idle, the Txd is kept
true, this helps the receiver detect when a sender is present. A single start
bit is sent by making the Txd false. In this example the next eight bits are
the transmitted data, a byte with the value 17. The data is followed by a
parity bit that can be used to check the byte. In this example there are two
data bits set, and even parity is being used, so the parity bit is set. The
parity bit is followed by two stop bits to help separate this byte from the
next one.

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125

Figure B.3 a serial data byte

Some of the byte settings are optional, such as the number of data
bits (7 or 8), the parity bit (none, even or odd) and the number of stop bits
(1 or 2). The sending and receiving computers must know what these
settings are to properly receive and decode the data. Most computers send
the data asynchronously, meaning that the data could be sent at any time,
without warning. This makes the bit settings more important.

Another method used to detect data errors is half-duplex and full-
duplex transmission. In half-duplex transmission the data is only sent in
one direction. But, in full-dup transmission a copy of any byte received is
sent back to the sender to verify that it was sent and received correctly.
(Note: if you type and nothing shows up on a screen or characters show
up twice you may have to change the half/full duplex setting.)

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The transmission speed is the maximum number of bits that can be
sent per second. The units for this are baud. The baud rate includes the
start, parity and stop bits. For example a 9600 baud transmission of the
data in Figure B.3 would transfer up to 800 bytes each second. Lower
baud rates are 120, 300, 1.2K, 2.4K and 9.6K. Higher speeds are 19.2K,
28.8K and 33.3K. (Note: When this is set improperly you will get many
transmission errors, or garbage on your screen.)
Serial lines have become one of the most common methods for
transmitting data to instruments: most personal computers have two serial
ports. The previous discussion of serial communications techniques also
applies to devices such as modems.

B.3 RS-232

The RS-232c standard is based on a low/false voltage between +3
to +15V, and an high/true voltage between -3 to -15V (+/-12V is
commonly used). Figure B.4 shows some of the common connection
schemes. In all methods the txd and rxd lines are crossed so that the
sending txd outputs are into the listening rxd inputs when communicating
between computers. When communicating with a communication device
(modem), these lines are not crossed. In the modem connection the dsr
and dtr lines are used to control the flow of data. In the computer the cts
and rts lines are connected. These lines are all used for handshaking, to
control the flow of data from sender to receiver. The null-modem
configuration simplifies the handshaking between computers. The three
wire configuration is a crude way to connect to devices, and data can be
lost.

Distributed Control System and Programmable Logic Control
127

Figure B.4 Common RS-232 Connection Schemes

Common connectors for serial communications are shown in
Figure B.5. These connectors are either male (with pins) or female (with
holes), and often use the assigned pins shown. The DB-9 connector is
more common now, but the DB-25 connector is still in use. In any
connection the RXD and TXD pins must be used to transmit and receive
data. The COM must be connected to give a common voltage reference.
All of the remaining pins are used for handshaking.

Distributed Control System and Programmable Logic Control
128

Figure B.5 Typical RS-232 Pin Assignments and Names

The handshaking lines are to be used to detect the status of the
sender and receiver, and to regulate the flow of data. It would be unusual
for most of these pins to be connected in any one application. The most
common pins are provided on the DB-9 connector, and are also described
below.
TXD/RXD - (transmit data, receive data) - data lines
DCD - (data carrier detect) - this indicates when a remote device is
present
RI - (ring indicator) - this is used by modems to indicate when a
connection is about to be made.
CTS/RTS - (clear to send, ready to send)
DSR/DTR - (data set ready, data terminal ready) these handshaking lines
indicate when the remote machine is ready to receive data.
COM - a common ground to provide a common reference voltage for the
TXD and RXD.

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129
Appendix C
Networking

C.1 Introduction

A computer with a single network interface can communicate with
many other computers. This economy and flexibility has made networks
the interface of choice, eclipsing point-to-point methods such as RS-232.
Typical advantages of networks include resource sharing and ease of
communication. But, networks do require more knowledge and
understanding.
Small networks are often called Local Area Networks (LANs).
These may connect a few hundred computers within a distance of
hundreds of meters. These networks are inexpensive, often costing $100
or less per network node. Data can be transmitted at rates of millions of
bits per second. Many controls system are using networks to
communicate with other controllers and computers. Typical applications
include;
 Taking quality readings with a PLC and sending the data to a
database computer.
 Distributing recipes or special orders to batch processing
equipment.
 Remote monitoring of equipment.
Larger Wide Area Networks (WANs) are used for communicating
over long distances between LANs. These are not common in controls
applications, but might be needed for a very large scale process. An
example might be an oil pipeline control system that is spread over
thousands of miles.

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C.2 Topology

The structure of a network is called the topology. Figure C.1 shows
the basic network topologies. The Bus and Ring topologies both share the
same network wire. In the Star configuration each computer has a single
wire that connects it to a central hub.

Figure C.1 Network Topologies

In the Ring and Bus topologies the network control is distributed
between all of the computers on the network. The wiring only uses a
single loop or run of wire. But, because there is only one wire, the
network will slow down significantly as traffic increases. This also
requires more sophisticated network interfaces that can determine when a
computer is allowed to transmit messages. It is also possible for a
problem on the network wires to halt the entire network.
The Star topology requires more wire overall to connect each
computer to an intelligent hub. But, the network interfaces in the
computer become simpler, and the network becomes more reliable.
Another term commonly used is that it is deterministic; this means that
performance can be predicted. This can be important in critical
applications.

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For a factory environment the bus topology is popular. The large
number of wires required for a star configuration can be expensive and
confusing. The loop of wire required for a ring topology is also difficult
to connect, and it can lead to ground loop problems. Figure C.2 shows a
tree topology that is constructed out of smaller bus networks. Repeaters
are used to boost the signal strength and allow the network to be larger.

Figure C.2 The Tree Topology

C.3 OSI Network Model

The Open System Interconnection (OSI) model in Figure C.3 was
developed as a tool to describe the various hardware and software parts
found in a network system. It is most useful for educational purposes, and
explaining the things that should happen for a successful network
application. The model contains seven layers, with the hardware at the
bottom, and the software at the top. The darkened arrow shows that a

Distributed Control System and Programmable Logic Control
132
message originating in an application program in computer #1 must travel
through all of the layers in both computers to arrive at the application in
computer #2. This could be part of the process of reading email.

Figure C.3 The OSI Network Model

Application - This is high level software on the computer.
Presentation - Translates application requests into network operations.
Session - This deals with multiple interactions between computers.
Transport - Breaks up and recombines data to small packets.
Network - Network addresses and routing added to make frame.
Data Link - The encryption for many bits, including error correction
added to a frame.
Physical - The voltage and timing for a single bit in a frame.
Interconnecting Medium - (not part of the standard) The wires or
transmission medium of the network.

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The Physical layer describes items such as voltage levels and
timing for the transmission of single bits. The Data Link layer deals with
sending a small amount of data, such as a byte, and error correction.
Together, these two layers would describe the serial byte shown in the
previous chapter. The Network layer determines how to move the
message through the network. If this were for an internet connection this
layer would be responsible for adding the correct network address. The
Transport layer will divide small amounts of data into smaller packets, or
recombine them into one larger piece. This layer also checks for data
integrity, often with a checksum. The Session layer will deal with issues
that go beyond a single block of data. In particular it will deal with
resuming transmission if it is interrupted or corrupted. The Session layer
will often make long term connections to the remote machine. The
Presentation layer acts as an application interface so that syntax, formats
and codes are consistent between the two networked machines. For
example this might convert ’\’ to ’/’ in HTML files. This layer also
provides subroutines that the user may call to access network functions,
and perform functions such as encryption and compression. The
Application layer is where the user program resides. On a computer this
might be a web browser, or a ladder logic program on a PLC.

Most products can be described with only a couple of layers. Some
networking products may omit layers in the model.

C.4 Networking Hardware

The following is a description of most of the hardware that will be
needed in the design of networks.
 Computer - (or network enabled equipment)

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 Network Interface Hardware - The network interface may already
be built into the computer/PLC/sensor/etc. These may cost $15 to
over $1000.
 The Media - The physical network connection between network
nodes.
10baseT (twisted pair) is the most popular. It is a pair of
twisted copper wires terminated with an RJ-45 connector.
10base2 (thin wire) is thin shielded coaxial cable with BNC
connectors.
10baseF (fiber optic) is costly, but signal transmission and
noise properties are very good.
 Repeaters (Physical Layer) - These accept signals and retransmit
them so that longer networks can be built.
 Hub/Concentrator - A central connection point that network wires
will be connected to. It will pass network packets to local
computers or to remote networks if they are available.
 Router (Network Layer) - Will isolate different networks, but
redirect traffic to other LANs.
 Bridges (Data link layer) - These are intelligent devices that can
convert data on one type of network, to data on another type of
network. These can also be used to isolate two networks.
 Gateway (Application Layer) - A Gateway is a full computer that
will direct traffic to different networks, and possibly screen
packets. These are often used to create firewalls for security.

Figure C.4 and C.5 shows the basic OSI model equivalents for some of
the networking hardware described before.

Distributed Control System and Programmable Logic Control
135

Figure C.4 Network devices and the OSI model

Figure C.5 The OSI network model with a router

Distributed Control System and Programmable Logic Control
136
Appendix D
Software Engineering

D.1 Introduction

A careful, structured approach to designing software will cut the
total development time, and result in a more reliable system.

D.2 Fail Safe Design

It is necessary to predict how systems will fail. Some of the
common problems that will occur are listed below.
Component jams - An actuator or part becomes jammed. This can be
detected by adding sensors for actuator positions and part presence.
Operator detected failure - Some unexpected failures will be detected by
the operator. In those cases the operator must be able to shut down the
machine easily.
 Erroneous input - An input could be triggered unintentionally. This
could include something falling against a start button.
 Unsafe modes - Some systems need to be entered by the operators
or maintenance crew. People detectors can be used to prevent
operation while people are present.
 Programming errors - A large program that is poorly written can
behave erratically when an unanticipated input is encountered. This
is also a problem with assumed startup conditions.

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137
 Sabotage - For various reasons, some individuals may try to
damage a system. These problems can be minimized preventing
access.
 Random failure - Each component is prone to random failure. It is
worth considering what would happen if any of these components
were to fail.

Some design rules that will help improve the safety of a system are
listed below.

Programs
 A fail-safe design - Programs should be designed so that they
check for problems, and shut down in safe ways. Most PLC’s also
have imminent power failure sensors; use these whenever danger is
present to shut down the system safely.
 Proper programming techniques and modular programming will
help detect possible problems on paper instead of in operation.
 Modular well designed programs.
 Use predictable, non-configured programs.
 Make the program inaccessible to unauthorized persons.
 Check for system OK at start-up.
 Use PLC built in functions for error and failure detection.

People
 Provide clear and current documentation for maintenance and
operators.
 Provide training for new users and engineers to reduce careless and
uninformed mistakes.

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138
D.3 Debugging

Most engineers have taken a programming course where they
learned to write a program and then debug it. Debugging involves
running the program, testing it for errors, and then fixing them. Even for
an experienced programmer it is common to spend more time debugging
than writing software. For PLCs this is not acceptable! If you are running
the program and it is operating irrationally it will often damage hardware.
Also, if the error is not obvious, you should go back and reexamine the
program design. When a program is debugged by trial and error, there are
probably errors remaining in the logic, and the program is very hard to
trust. Remember, a bug in a PLC program might kill somebody.

D.4 Troubleshooting

After a system is in operation it will eventually fail. When a failure
occurs it is important to be able to identify and solve problems quickly.
The following list of steps will help track down errors in a PLC system.
Look at the process and see if it is in a normal state. i.e. no jammed
actuators, broken parts, etc. If there are visible problems, fix them and
restart the process.
1. Look at the PLC to see which error lights are on. Each PLC vendor
will provide documents that indicate which problems correspond to
the error lights. Common error lights are given below. If any off
the warning lights are on, look for electrical supply problems to the
PLC.
a. HALT - something has stopped the CPU
b. RUN - the PLC thinks it is OK (and probably is)
c. ERROR - a physical problem has occurred with the PLC

Distributed Control System and Programmable Logic Control
139
2. Check indicator lights on I/O cards, see if they match the system.
i.e., look at sensors that are on/off, and actuators on/off, check to
see that the lights on the PLC I/O cards agree. If any of the light
disagrees with the physical reality, then interface
electronics/mechanics need inspection.
3. Consult the manuals, or use software if available. If no obvious
problems exist the problem is not simple, and requires a technically
skilled approach.
4. If all else fails call the vendor (or the contractor) for help.

D.5 Forcing

Most PLCs will allow a user to force inputs and outputs. This
means that they can be turned on, regardless of the physical inputs and
program results. This can be convenient for debugging programs, and, it
makes it easy to break and destroy things! When forces are used they can
make the program perform erratically. They can also make outputs occur
out of sequence. If there is a logic problem, then these don’t help a
programmer identify these problems.

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140
References

1. I/A series Foxboro documentation.
2. HoneyWell Experion process knowledge system, "Honeywell
Training.ppt"
3. "Automation Hierarchy", By: Prof. Dr. H. Kirrmann, ABB
Research Center, Baden, Switzerland, " AI_14_Hierarchy.ppt"
4. http://newton.ex.ac.uk , By: C.D.H. Williams
5. "Electrical Relay Diagram And P&ID Symbols", From Industrial
Text and Video Co. The Leader in Electrical, Motor Control and
PLCs Video Training Programs (www.industrialtext.com).
6. "A PLC Primer", " www.industrialtext.com".
7. "Automating Manufacturing Systems with PLCs", By:" Hugh
Jack" ([email protected]).
8. "Regulatory and Advanced regulatory control system
development", By: Harold L. Wade, Instrumentation society of
America.
9. Rosemount Measurement Catalog.

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