ETL 1110-2-553
30 May 97
techniques. This made the cumbersome relay panels
control voltages to nonlethal levels, typically 24 volts.
and mazes of wire and cabling obsolete, similar to the
Also, because signals between the I/O cards and the
obsolescent mechanical calculators in favor of elec-
CPU can be digitally multiplexed, the signals can be
tronic computers.
sent across one fiber-optic cable, eliminating the need
for the numerous long cable runs and raceways found
(b) As microprocessor technology evolved, the
in the relay-based systems.
PLC capabilities were upgraded to take advantage of
these advances. PLC's were no longer just relay
(c) The latest PLC systems have the ability to
replacers. They became capable of accepting a wide
directly communicate (without the I/O modules) with
variety of feedback electrical signals which permitted
traditional devices such as motor control centers,
them to monitor and control electrical equipment such
transfer switches, power and lighting panels, AC and
as motors, temperature sensors, pressure sensors,
DC motor drive systems, position tracking equipment,
vibration sensors, motion detectors, lights, horns,
and IPC control stations. The trend in the industry is
solenoids, etc.
toward integrated and intelligent power distribution
and control systems.
(c) In the 1980's, software was developed which
permitted the PLC to communicate with personal
(d) The PLC can be programmed through the
computers. The merging of these two devices revolu-
use of the IPC. An advantage to this method of
tionized operator interfaces. Hardwired control pan-
accessing the PLC is that, through the use of another
els with pushbuttons, switches, lights, and meters
personal computer and a modem, an engineer in the
became obsolete in favor of computer-screen images
district office, or anywhere for that matter, can assist
of these devices. The PLC's communicate with the
field maintenance personnel when changes or trouble-
computers over local area networks using digital
shooting are necessary.
signals transmitted over fiber optics. Operators can
now perform control actions by clicking on an image
(e) The programming language for the PLC was
with a mouse or using a touch screen. The personal
specifically designed for electrical engineers and
computers can be hardened for greater reliability and,
electricians in the industrial controls field. The lan-
if so, are referred to as industrial personal computers
guage is composed of symbols similar to those the
(IPC's). Use of computers has improved flexibility
engineer and electrician were accustomed to seeing in
since changes to operating instructions and control
conventional relay ladder schematics. The concept of
configurations can be implemented by simply pro-
a relay and its contacts was maintained. The only
gramming new computer code. No physical changes
difference is that, instead of being a physical relay, it
need be made to the system. Reprogramming can be
now exists only as a software relay internal to the
accomplished offsite with new operating code trans-
PLC and can be displayed graphically on the comp-
mitted to the IPC by telephone modem.
uter monitor.
(2) Description.
(3) Evaluation.
(a) The PLC consists of two sections: the cen-
(a) PLC systems have emerged as the system of
tral processing unit (CPU) and the input/output (I/O)
choice for industrial control systems of any complex-
modules. Logic operations and arithmetic manipula-
ity. The PLC has proven superior to relays in all but
tions are performed in the CPU just as in any other
the simplest systems. Systems with as few as five or
computer. Communication with the electrical field
ten relays can usually be economically replaced with
devices is through I/O cards located on the I/O mod-
PLC technology. Control systems at locks and dams
ules. A variety of plug-in cards are available to
far exceed the minimum criteria for economical appli-
accommodate the different types of electrical signals
cation. The PLC easily and economically permits the
encountered.
development of control systems of virtually unlimited
complexity. Complex systems are developed by
(b) The I/O cards are where all the thousands of
typing in code on a keyboard and can be repro-
conductors from the field devices are terminated.
grammed in a matter of keystrokes.
Since the I/O cards can be remotely located from the
CPU, and are usually strategically located around the
(b) There is no perfect physical system, how-
facility to be controlled, it is possible to reduce
ever. PLC's have their own unique vulnerabilities.
A-5
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