Advantech ADAM-5511 Manual page 95

The Up/Down Counter Function offers two types of counting:
Up Couting (increasingly) and Down Counting (decreasingly).
Up Counting : when C0A+ and C0A- sense any input signals, the
counter counts up.
Down Counting : when C0B+ and C0B- sense any input signals, the
counter counts down.
On receiving Up and Down signal simultaneously, the counter will not
perform each specific counting accordingly, but will remain at the
previous counting value, since these simultaneous signals won't have
any effect on counting values.
Note: If you need only one type of counting, connect C0A+
and C0A- for Up Counting only; or connect C0B+
and C0B- for Down Counting only.
For implementing Bi-derection Counting, you need to connect C0B+/
D+ and C0B-/D- to implement the control function for
Up/Down Counting.