Counters are PLC instructions that either increment or decrement or decrement the integer value when the input line make True from False. The counter which increment value is known as Up counters and Down counters decrement the integer values on a trigger. Both the Up and Down counter starts counting on one trigger.
And there is an Up-Down counter which does both the Up and Down counting. The Up-Down counter has two input triggers one for Up counting and other for Down counting.
Up counter:
Up counter counts a value from the current value when a signal arrives at the input CU. R is the reset input and PV the preset value. The counter counts up until the count reaches the preset value.
Whenever the input CU value changes from 0 to 1 the counter increment 1 from the preset value. When the input value reaches the preset value the output turns ON. The counter preset value is user defined.
Down counter:
Each time the input CD turns ON the counter decrement one from the preset value. Whenever LD (load ) input becomes high then the preset value is reloaded to the counter from the current value.
Whenever the input CD value changes from 0 to 1 the counter decrement 1 from the preset value.
Up/Down counter:
The Up/Down counter work as both the up counter and the down counter. The up/down counter has input for both the Up counter and Down counter operation CU and CD. And has both the inputs R and LD also.
This type of counter has two output values too. one for the CD and other for CU. R resets the PV value to zero and LD load the load the preset value to the counter ignoring the current counter value.
The two input value has two corresponding output lines for CD and CU.