s, have been used in industry in one form or another for the past fifteen years. The newest models may not resemble their earlier counterparts, but many of the concepts used in the first units are still in use. The programmable controller is basically a computer-controlled system containing a microprocessor that is programmed with a programming panel, or keyboard. The P.C. is dedicated to receiving input signals and sending output signals in response to the program logic. The program generally consists of contacts, outputs, timers, counters, and math functions. The programmable controllers found in industry today have evolved from the need for a control system that can be easily reprogrammed as changes occur or as new products develop.
For example, the automotive industry is faced annually with major changes in production as new models are designed. This changeover previously required electricians and maintenance personnel to put in long hours to rewire relay-type controls. Each changeover period was costly to the industry, and it often forced changes to be infrequent and as simple as possible.
The programmable controller was developed in 1969 to ease the problem of changing control systems periodically. Modern P.C's consist of four major parts:.
a central processing unit, also known as the processor or computer; an input section; an output section; and a power supply (see Fig. 1-1). The programming panel is not considered a major part of an operating P.C. because once you have used it to program the processor, it can be disconnected and the system will still operate correctly. The relationship of these four parts and the ability of the P.C.'s to be reprogrammed can best be understood by comparing hardwired circuits to identical circuits controlled by a P.C.
In the P.C. -controlled system, electrical power comes from the voltage source, through the switch into the input module. The input module senses the presence of this voltage and inturn, sends a small signal voltage into the processor.
