Siemens PLC counting instructions

Each time the Siemens PLC's up count input switches from off to on, the up count (CTU) counts up from the current value of the counter. The counter is reset when the reset input turns on or a reset instruction is executed. The counter stops when the maximum value (32,767) is reached.

The up/down counter (CTUD) counts up each time the up count input performs an off-to-on transition, and each time the down count input performs an off-to-on transition (32,767), the next rising edge of the up count input causes the current count to reach the minimum value (32,768). Similarly, when the minimum value (-32,768) is reached, the next rising edge of the down count input causes the current count to reach the maximum value (32,767).

The up and up/down counters have a current value that holds the current count. The counters also have a preset value (PV) that is combined with the preset value each time the counter instruction is executed when the C bit is on. Otherwise, the C bit is off.

The down counter (CTD) counts down from the current value of the counter each time the count down input performs an off-to-on transition. When the load input turns on, the counter resets the counter bit and loads the preset value into the current value. When zero is reached, the counter stops and the counter bit (C bit) turns on.

When you use the Siemens PLC reset command to reset a counter, the counter bits are reset and the counter current value is set to zero. Use the counter number to reference the current value and C bit of the counter.

Notes:

Because each counter has a current value, do not assign the same number to more than one counter. (Up counters, up/down counters, and down counters access the same current value.)