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Programmable Logic Controllers, 5th Edition by William Bolton

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Chapter 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 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. Normally open contacts would be closed, normally closed contacts opened.
There are two basic types of counter: down-counters and up-counters. Down-counters count down from the preset value to zero, that is, events are subtracted ...

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