Programmable logic controllers (PLCs)
With complex control requirements, the use of electro-mechanical
relays is somewhat cumbersome, and most modern systems employ
PLCs. In basic terms these do no more than relays, i.e.

they process
the input information and activate a corresponding output. Their
great advantage, however, is in the use of microelectronics to achieve
the same end. The saving in space and low failure rate (there are no
moving parts) make them very desirable. A typical unit for, say, 20
inputs (I) and 20 outputs (O), referred to as a 40 I/O unit, would
measure approximately 300 mm by 100 mm by 100 mm, and would
also incorporate counters, timers, internal coils etc.
A PLC is programmed to function in a specified way by the use
of a keyboard and a display screen.

The information may be
programmed directly into the PLC, or a chip known as an EPROM
may be programmed remotely and then plugged into the PLC. The
programming method uses ‘ladder logic’. This employs certain
symbols, examples of which are shown in Figure 14.

These symbols
appear on the screen as the ladder diagram is built up.