I have inherited this gantry style robot used in the PCB industry and am trying to understand better its components. I am familiar with the motion and PLC programming aspect of it but would like to better understand the digital I/O circuitry.

Its controlled by a Compumotor 6K.

What I’m really interested in is the relay terminals and why and when to use them.
They are connected to the TTL inputs and outputs of the 6K (5Volt DC)
Most of them are similar to Allen-Bradley Terminal Block Relays 700-HL series)

Can someone recommend a book. or other media, for beginners that might explain the use of these in robotics control systems. And digital I/O wiring in general.

Thanks
TPC