never having used MACH3 I dont know... but I guess its the same as setting up the estop - it should act the same, and lo, page 4 of the MACH3 manual (here on Keling's site) shows how to do this - on the inputs tab, select it as pin 11 and active low checked. However that raises an issue, since MACH3 only supports 1 estop input, so how would the estop switch be configured? Well one option would be to wire it in series with the servo alarms. I think however there is a simpler and better answer... dont wire the estop switch to MACH3.

My reasoning is as follows: I'd not want rely on MACH3 to do the estop function, I'd want 100% certainty that when I hit the switch it stops things dead by killing power to the whole device. In which case I'd want a relay/contactor on the power input to the drives and to the spindle which was opened when I hit estop.

If the servo alarms are fed back to MACH3 as described they will open when the power is killed and tell MACH3 there is a problem so that works correctly.

Hope that helps...