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When logic circuits become very large, such as in the control of large
industrial machinery, it becomes necessary to change the interaction
between logic gates from time to time to change the function of the machine
control. In the late 1960s and early 1970s, several companies each designed
a programmable logic controller (PLC), which is now known as the programmable
controller. These early companies included Modicon, Allen-Bradley, and
Texas Instruments, and the controllers they designed were made specifically
for General Motors. The programmable controllers represented the logic
functions with contacts and coils symbols, much like the original relay
logic. These symbols are used instead of logic symbols because the electricians
and technicians that use the programmable controllers are used to using
relay logic. In fact most of the early PLCs were used as relay replacements
because they were very cost-effective in providing complex machine control
through logic circuits. The PLC provides several other major advantages
over logic chips in that they could be reprogrammed easily, they could
be troubleshooted with a video screen that showed which contacts are open
or closed, and their logic circuit (program) could be stored and downloaded
into the machine control at a later date.
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Today PLCs are available as the simple, low-cost replacement for relay
logic, or to provide complex functions such as data storage, file manipulation,
and process control. Each of these topics will be covered in detail
in later s ections of this book.
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