PLCs-4e-A&S 06-04, 06-07, 06-12-2013
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counting, and arithmetic functions through digital or analog input/output (I/O) modules, for
controlling machines and processes.
9.10 What are the advantages of using a PLC rather than conventional relays, timers, counters,
and other hard-wired control components?
Answer: The advantages listed in the text are (1) programming the PLC is easier than
wiring the relay control panel; (2) the PLC can be reprogrammed, whereas conventional
controls must be rewired and are often scrapped instead; (3) PLCs take less floor space
than relay control panels; (4) reliability is greater, and maintenance is easier; (5) the PLC
can be connected to computer systems more easily than relays; and (6) PLCs can perform a
greater variety of control functions than relay controls can.
9.11 What are the four basic components of a PLC?
Answer: The four basic components of a PLC are the following: (1) processor, (2) memory
unit, (3) power supply, and (4) I/O module. In addition, a programming device is required
to enter the control program to the PLC. In most cases, this can be disconnected from the
PLC when not being used.
9.12 The typical operating cycle of the PLC, called a scan, consists of three parts: (1) input
scan, (2) program scan, and (3) output scan. Briefly describe what is accomplished in each
part.
Answer: During the input scan, the inputs to the PLC are read by the processor and the
status of each input is stored in memory. Next, the control program is executed during the
program scan. The input values stored in memory are used in the control logic calculations
to determine the values of the outputs. Finally, during the output scan, the outputs are
updated to agree with the calculated values.
9.13 Name the five PLC programming methods identified in the International Standard for
Programmable Controllers (IEC 61131–3).
Answer: The standard specifies three graphical languages and two text-based languages
for programming PLCs, respectively: (1) ladder logic diagrams, (2) function block
diagrams, (3) sequential functions charts, (4) instruction list, and (5) structured text.
9.14 What are the reasons and factors that explain why personal computers are being used with
greater and greater frequency for industrial control applications?
Answer: Some of the reasons given in the text are the following: (1) The technological
evolution of PLCs has not kept pace with the development of PCs. (2) New generations of
PCs are introduced with much greater frequency than PLCs. (3) There is much more
proprietary software and architecture in PLCs than in PCs, making it difficult to mix and
match components from different vendors. (4) PCs are now available in more-sturdy
enclosures for the dirty and noisy plant environment. (5) PCs can be equipped with
membrane-type keyboards for protection against factory moisture, oil, and dirt. (6) PCs can
be ordered with I/O cards and related hardware to provide the necessary devices to connect
to the plant’s equipment and processes. (7) Operating systems designed to implement real-
time control applications can be installed in addition to traditional office software.
9.15 Name the two basic approaches used in PC-based control systems.
Answer: The two approaches are soft logic and hard real-time control. In the soft logic
configuration, the PC’s operating system is Windows, and control algorithms are installed © 2015 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication is protected by Copyright and written permission should be obtained from
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