Computers and industrial control Figure The component parts of a PLC system: (a) an early PLC system; (b) a typical rack of cards Figure (b) which is identical to the relay circuit needed to control the cylinder. These programs look | Computers and industrial control 19 Cassette recorder Cable linking processor to racks Programming terminal Central processor Power supply-for rack Power supply cable Interface Rack for card I o card 81 0 cards b Figure The component parts of a PLC system a an early PLC system b a typical rack of cards Figure b which is identical to the relay circuit needed to control the cylinder. These programs look like the rungs on a ladder and were consequently called ladder diagrams . The program was entered via a programming terminal with keys showing relay symbols normally open normally closed contacts coils timers counters parallel branches etc. with which a maintenance electrician would be familiar. Figure shows the programmer 20 Programmable Controllers Figure A simple PLC application. a A simple hydraulic cylinder controlled by a PLC. b The ladder diagram program used to control the cylinder. This is based on American relay symbols. - - means that signal is present and means that signal is not present keyboard for an early PLC. The meaning of the majority of the keys should be obvious. The program shown exactly on the screen as in Figure b would highlight energized contacts and coils allowing the programming terminal to be used for simple fault finding. The processor memory was protected by batteries to prevent corruption or loss of program during a power fail. Programs could be stored on cassette tapes which allowed different operating procedures and hence programs to be used for different products. The name given to these machines was programmable controllers or PCs. The name programmable logic controller or PLC was also used but this is strictly a registered trademark of the Allen Bradley Company. Unfortunately in more recent times the letters PC have come to be used Computers and industrial control 21 Figure The programming terminal keypad for an early Allen Bradley PLC reproduced by permission of Allen Bradley for personal computer and
