You could also do it without a relay using a DPDT estop button. Use one side to disable the drives and use the other side, wired NC, as an input to Mach. Set the input as active high and when the button is pushed and the switch opens, mach triggers an estop, while at the same time your drives shut down. I agree with Hood in that high voltage relays are better in a noisy environment. Never had noise issues though, even running HF plasma.
I started using the G540 drives and this functionality is built into the drive. When I disable the drive with an external estop, it sends an estop request to mach. I think that is built into their internal BOB.