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So, change made, AC power component is gone and first small test job ran successfully. Hoping for the best . Thank you again!

Ah-ha! The 24V field power GND is not connected to earth ground, I was not sure that it should be. That probably explains it then.

Just weird that the machine ran fine for so long.

Thanks, I was getting frustrated with the constantly changing symptoms. I will connect those grounds tomorrow and test again.

Ack!! 40V is a real issue and indicates a grounding problem. This likely
what damaged the step/dir outputs (accidental contact to shield ground would do it)

I would verify that the drive frame grounds connect to earth ground and that the
7I76E ground (24V logic power ground) is also connected to earth ground.

Thanks a lot for your help!

Now that I hooked up an oscilloscope, the problem disappeared completely again - can't reproduce it anymore, at all.

That made generating pulses easier of course, I simply jogged the thing along. The levels of the step+/- pins on the motor side are around 4.8 volts, and -4.8 on a pulse.

I also noticed that the level between STEP- and the cable shield (GND) is around 40 volts AC (50 Hz). I can't say I understand why that would be so. Is that normal/intended?

Perhaps an output is marginal because of previous damage

I would verify full 5V swing on all step/dir pins.

Here is a simple script to toggle all stepgen step and direction pins slowly:

 


Download then

chmod +x stepgens.txt

and

./stepgens.txt to run

Note that this needs a relatively recent mesaflash

github.com/LinuxCNC/mesaflash

Alright, I found out that though the error looks identical, this time the MESA card is NOT the problem. The other good news is that the problem has become reproducible and the Joint Amplifier Failure now always happens immediately during the (attempted) homing procedure.

I can see during the homing attempt that the joint that has the amplifier error attempts to move - the other joint does not. In other words, I think that the servo with the alarm is the good one, and it goes into alarm because it fails to drag the other servo along.

It is still a puzzle though. By swapping port/cables/motors it seems that the common denominator is a port/cable:

 

The red path marks the one that has amplifier error in the different scenarios that I tested.

What puzzles me is that the common denominator in all failure scenarios are panel ports 2 and 3. However, I have measured the every wire several times with an ohm meter (as shown by the green lines in the image), and they are all fine. Since they are also not moving cables I cannot imagine it is a flapping connectivity issue.

So I am a little clueless as to what else I could do now. It seems every potential source of the problem was excluded.

Maybe EMI could have something to do with it, but... why would that suddenly become a reliably reproducible problem after running years with no issue?
Anyway, I also measured all the grounds on the shields of each cable, and they also seem fine/low impedance. (star configuration off of the panel, no loops)

Any ideas what else I can check?

Yes, if the touch probe uses a contact to ground you can use a pull-up
resistor as Andy suggested.

For 7I76XX inputs this needs to be fairly stiff, say 2.2K 1/2W

This is because the 7I76XX input resistance is 11K and needs an input > 14.4V
for a high level at 24V Vfield, and a 10K resistor will not do this.

One way to use a sinking input with a switch-to-ground sensor is to put a resitstor (I would probably try a 10k, choose something that can handle the current that flows when connected between 24V and gnd, anything over 2.5k @ 1/4 W size will be OK). Put one leg of the resistor in the 7i76 input, and connect the other to 24V, The connect the leg of the resistor that is in the input terminal to your switch-to-gnd.
With the switch closed the voltage at the input will be zero, with it open the resistor will pull-up the input and set it true in HAL.

You can invert the sense of the input in the HAL layer to get the right polarity,