Thanks. I can't think of anyone around with a scope. And it would take a long time of watching (or recording) the data to see which one misses the step.

Thanks tripleback. That is my solution. They are not that expense and can be swapped out quickly. Over the past couple of days I have made some expense firewood which costs far more than a C4. I have already ordered some digital sensors so I can restart with accuracy. See http://www.cnczone.com/forums/open_source_cnc_machine_designs/101878-electronic_home_switches_made_easy.html for details.

Thanks for the replies. Dave

You can use an earphone to detect the 12.5khz signal but detecting dropouts or intermittent errors is out.
If it fails for some reason such as a software bugger-up, will it stop altogether or recover itself?

Thanks BluePinnacle. It is easier to replace than troubleshoot. However, I revisited the M3 XP_optimization page and rechecked everything. All that I did was a disk cleanup, a defragment and reboot. It has ran perfectly for the past three days. Go figure. It dawned on me that since I have a tape on the X axis, I could simply write down the X stating zero position to within 1/64". And on Y, a piece of blue tape across the track at the zero position. Its eyeball accuracy but usually close enough for woodworking. Still waiting on the magnetic digital switch to arrive.