Forms of Counter - Programmable Logic controllers

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 from the set value. When the counter reaches the zero value, its contacts change state. Most PLCs offer down-counting.

Up-counters count from zero up to the preset value, that is, events are added until the number reaches the preset value. When the counter reaches the set value, its contacts change state.Some PLCs offer the facility for both down- and up-counting. Figure shows the IEC symbols for such counters.

Different PLC manufacturers deal with counters in slightly different ways. Some count down (CTD) or up (CTU) and reset and treat the counter as though it is a relay coil and so a rung output.

In this way, counters can be considered to consist of two basic elements: one relay coil to count input pulses and one to reset the counter, the associated contacts of the counter being used in other rungs. Figure llustrates this method. Mitsubishi is one of the manufacturers that uses this method. Others treat the counter as an intermediate block in a rung from which signals emanate when the count is attained. Figure illustrates this method, used by Siemens, among others.

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