First, you mention "106" a couple of times. I presume that you mean "126" (as in PMDX-126) since you don't list a PMDX-106 as one of the boards that you have.
Second, is this a continuation of the issues from this thread?
http://www.pmdx.com/PMDX-Forums/index.php?topic=43.msg415#msg415(for which I apologize for not responding to your last post).
When you press one of your safety switches, do you see the red EStop LED on the PMDX-126 turn on? When you release the safety switch, does the red EStop LED on the PMDX-126 turn off? IF not, then something is keeping the EStop input high (or open) or the Fault input grounded.
Let me see if I have this straight: when this issue arises, you can turn the PMDX-126 off and back on and the problem goes away (i.e.you can bring Mach out of reset). **OR** you can exit Mach and re-start it and you are also then able to bring Mach out of reset. Is that right?
You mentioned in the other thread that things worked for a while, then stopped working. This *may* indicate intermittent connections. Try removing one wire at a time from the PMDX-126, then make sure that the screw clamp is all the way down, re-insert the wire that tighten the screw clamp.
Are you using one of the PMDX-126 relays to also control power to the motors/PMDX-134 (as in our app note AN003)? If so, you have a deadlock condition because when the PMDX-126 sees an EStop/Fault input it will turn off the relay, cutting power to the motors, which will then keep the fault input grounded through the power monitor relay.
Try disconnecting the motor power from the PMDx-126 fault input and see if that changes things.
Bob