Solution:

The detailed descriptions about control logic of 785B is shown as follows: Positive logic negative logic 20-pin flat cable connector V -- 50-pin Opto-22 connector * V Note: Positive logic => Input TTL high (+5V) = Relay on Negative logic => Input TTL low (+0V) = Relay on * => The default control logic of 785B for opto-22 connector is negative.