A counter in PLC is an instruction equivalent to mechanical counter. A counter block of PLC has two control inputs, Count line and Reset line. When count line makes FALSE-to-TRUE transition, the counter increments/decrements its accumulated value by 1. FALSE-to-TRUE transition on reset line resets the accumulated count to zero. The common format of counter instruction of PLC is shown in following figure.
Commonly used counters used in PLC are 1) UP Counter 2) DOWN counter 3) UP-DOWN Counter The counter operation is shown with the help of timing diagram of an up-down counter which is combination of up and down counter. The up-down counter has three inputs, count up, count down and reset.