Well. Which mach3 board do you have?.
In terms of your question.....
You want a suitable SCSI cable for CN1 connection.
Once you've got that you are looking for the connections marked.... 'F-PC and R-PC'.
If I have the right manual it shows on page '3-16' the differential or open collector wiring diagram.
A Mach board capable of 'differential' is preferable with servos (if using 150khz+ pulse rates).
'Open collector' wiring is same like you'd find on stepper drives if using 100khz or less.