Advantech ADAM-5510/P31 Manual page 109

Up Counting : when the input signal is within logic level "1", the
counter value increases.
Down Counting : when the input signal is within logic level "0", the
counter value decreases.
Note: If users select TTL mode and don't connect C0B+
C0B-, the counter value will increase. If users select
Isolated mode and don't connect C0B+ C0B-, the
counter value will decrease.
If users want to select frequency mode, they can only utilize Up
Counting type, and can only connect to C0A+ and C0A-.