Conventional industrial machine controls use hard-wired relays, timers and counters. With this method, however, the functionality of the circuit cannot be tested until all wiring has been completed.
Any modification to the circuit will be time consuming and difficult. The greatest disadvantage of this type of system is that it requires frequent maintenance because
of the short service life of each component.
Programmable logic controllers were developed to solve such problems. Since the control process of programmable
logic controllers is created with programs, control elements can be changed in the programs. PLCs also
perform arithmetic operations and control high-speed lines, which cannot be performed using a combination of
relays, timers and counters. Since semiconductor devices and microcomputer technology have been
rapidly advancing in recent years, PLCs now have a much longer service life. Therefore, little maintenance is required. |
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and a
relay output is graphically shown as
. PLCs must convert the ladder program symbols to mnemonics which then can be processed. The ladder program above can be represented by the following mnemonic words.
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