I have Z home enabled. X and Y are disabled. I have soft limits of +/-100,000. The input switch is not active.

I hit the Ref All Home button and the Z axis moves +.200 and stops. The Z DRO resets to whatever the initial value was. That is, if it started at -2.00, it travels .2, stops @-1.8  and resets the DRO to -2.00. No error message.

What am I doing wrong?

In Config >Homing/Limits, do you have a Home Offset of -2?
If so, then noise is probably triggering your switch.

The home offsett is zero. I don't think it is noise as the behavior is consistent.  I can have the Z anywhere on its travel and have the DRO set to any number. It always moves +.2 and stops.

please attach your xml.
Hood

Thanks for the offer Hood. It was operator error. The button had the legacy DoButton code. I switched it to DoOEMButton and it worked.

Strange  the DoButtons should work fine but as long as you have it working thats all that matters
Hood

It's running through a DSPMC. That may have something to do with it.

ah ok. Is it version 066 of Mach by any chance?
Hood

Yes, it is R3.043.066

Well there are known issues with it and homing, most likely when using external controllers. I have gone back to 056 or 057 .
Hood