Counters in PLC Types of PLC Counter

Counters in PLC | Types of PLC Counter

 Counters count rung transitions.  The CTU runs the accumulated value of the counter up on the false to true rung transition, and the CTD instruction runs the accumulated value down.  The CTU and CTD can be used in conjunction with each other. Counters consist of the following components: ACC Accumulated Value PRE Preset Value CD Count Down Bit CU Count Up bit OV Overflow Bit UN Underflow bit By default, data file C5 stores counters, however, other counter files can be added as well. Below is how the C5 Data file would appear:

For the CTU instruction:  The CU bit is high when the CTU instruction is true.  The ACC value increments by the value of 1 each time the CU bit goes high.  When the ACC  reaches the PRE, the DN bit will be set.  The CTU will continue to increment the  accumulated value until it reaches the maximum possible value for a 16 bit signed integer  (32767).  If the CU bit goes high one more time, the OV bit will be set, and the ACC value will go to -32768.  Each time the CU bit goes high, the ACC value will still continue to increment (become less negative). 

For the CTD instruction:  The CD bit is high when the CTD instruction is true.  The ACC value decrements by the value of 1 each time the CD bit goes high.  Any time the ACC  is above or equal to the PRE, the DN bit will remain set. The DN bit is reset if the ACC falls below the PRE at any time.  The CTD will continue to decrement the accumulated value until it reaches the minimum possible value for a 16 bit signed integer (-32768).  If the CD bit goes high one more time, the UN bit will be set, and the ACC value will go to 32767.  Each time the CD bit goes high, the ACC value will still continue to decrement (become less positive).

Here is a practical example of a CTU/CTD implementation:

 

Each time a pizza goes into the oven, the ACC value is incremented by one.  Each time a pizza comes out of the oven, the ACC value is decremented by one.  Therefore, the ACC value represents how many pizzas are in the oven at any given time.  The DN bit could be used to shut the conveyor down if pizzas are going into the oven and not coming out!