10 Counters
Counters are provided as built-in elements in PLCs and allow the number of occurrences of input signals to be counted. This might be where items have to be counted as they pass along a conveyor belt, or the the number of revolutions of a shaft, or perhaps the number of people passing through a door. This chapter describes how such counters can be programmed.
10.1 Forms of counter
A counter is set to some preset number value and, when this value of input pulses has been received, it will operate its contacts. Thus normally open contacts would be closed, normally closed contacts opened.
There are two types of counter, though PLCs may not include both types. These are down-counters and up-counters. Down-counters count down from the ...
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