One step at a time

If you look directly into the DB25 pins on the connector you can see the numbers there.
1 output
2 step x
3 dir x
4 step y
5 dir y
6 step z
7 dir z
8 step a
9 dir a
10 input
11 input
12 input
13 input
14 output
15 input
16 output charge pump
17 output
One of the inputs is the E-stop, the others are normal inputs
One or two of the outputs go to transistors inside the G540 to drive relays
Sorry but i forgot exactly where and what the rest does, it was 7 or 8 years since i had a G540

I use the DS series of LAM Technologies drives, pricey but by far the best stepper drives.