Up/Down Counter
The Count Up/Down instruction (CTUD) counts up or down
from the current value (CV) on the rising edges of the Count
Up (CU) or Count Down (CD) input. When the current value
is equal to preset, the up output (QU) turns on. When the
current value is equal to zero, the down output (QD) turns
on. The counter loads the current value with the preset
value (PV) when the load (LD) input is enabled. Similarly,
the counter resets and loads the current value with 0 when
the reset (R) is enabled