First test:
1) start Mach4 with your PMDX-411 profile, then bring up the PMDX real-time display (Diagnostics menu, PMDX-SmartBOB, then click on "real time display"). For this step it does not matter how you have Mach4 configured other than having the PMDX-411 selected as the "motion device".
2) Turn on G540 and plug the PMDX-411 into the parallel port (if not already).
3) Place a jumper wire on the G540 between terminal block pins 10 and 12. This should cause the G540 red LED to turn off. Look at the Mach4 PMDX real-time display for pin 15. The indicator should be green (I think).
4) Remove the jumper wire from the G540 terminal block pin 10. The G540 red LED should turn on, and the real-time display for pin 15 should change to red (I think, if it was red before, it should now be green)
If you don't see that, there is a hardware issue between the PMDX-411 and the G540.
Second test:
1) In Mach4, in the configure, Mach4, Input Pins tab, configure the E-Stop input (NOTE: **NOT** "Input #15") to be PMDX-411 pin 15. There should be a red "X" in the "active low" column. Click on OK to save any changed
2) With the wire still disconnected from the G540 terminal block pin 10, you should not be able to bring Mach4 out of EStop (cannot enable it). I think you may see a message about EStop still active, or something like that.
3) Re-connect the wire on the G540 terminal block pin 10 to pin 12.
4) You should now be able to enable Mach4.
5) And if you again disconnect the wire from G540 terminal pin 10, mach4 should disable itself.