This unit isolates connections to protect your computer from faults that could occur e.g. short-circuit’s.  An opto-isolator is an integrated circuit that transmits the signal through an encapsulated LED and phototransistor.  When the signal is on, the LED lights up, the phototransistor captures it and relays the signal.  In that way, your computer’s electronics are completely isolated from the cnc circuitry. The signals are transmitted through light and not through physical connections.  In this way, a power surge has no way of reaching your computer from your steppers drives.

They are 17 X 41 X 30.5mm in size.  They are very easy to install.