"7i92+7i76 on P2: SSerial detects board but registration fails with EAGAIN"

May be an introduced PathPilot bug since their hostmot2 driver has been
changed from LinuxCNC's version and Tormach doesn't use any sserial devices.

When the error happens, do you get a red LED illuminated on the 7I92?
 

From the ai...because my brain won't retain Linux. 
That's a genuinely significant lead — if PathPilot's Tormach-modified hostmot2 driver never gets tested against SSerial hardware at all (since Tormach's own machines don't use it), it's entirely plausible they introduced a real bug in that code path that nobody would have noticed. That would explain everything we hit today far better than any config mistake on our end.On the red LED question — I can't answer that myself, but here's the thing: we're not currently running the SSerial-enabled bitfile right now (we're back on the plain x1D raw-GPIO config for this E-stop pin testing). So that check will need to wait until we go back to testing the x2D/SSerial setup again. Worth flagging to PCW in your reply that you'll check and report back once you re-test that configuration — and it's a good, easy thing to note for next time (literally just look at the 7i92 board's LEDs during the failure).
And a dumb question from me, is it possible to revert to the Linuxcnc hostmot2 driver? I'm guessing not. Thanks!

This is getting aggravating, you should be capable of checking a simple LED and reply here with yes or no.
Power the Mesa board, start LinuxCNC, look at the Mesa board, does a red LED turn on?
That is NOT Linux, nor something ChatGPT can answer for you.

We, me and the ai, tripped over what may be the root cause. I will have better power supplies tomorrow and report back. Thanks for your time and attention!

Found it — the 5V logic supply was sagging to ~4.08V under load (was running off a 2A phone charger feeding both the 7I92 and 7I76 through the cable). Swapped to a proper 5V/7A supply, and GPIO pins now show a clean, full swing (5.09V/0V) where they were previously stuck flat regardless of software commands.Credit to tommylight and PCW — you both flagged marginal 5V power as a suspect early on, and that's exactly what this turned out to be.

Going to retest the SSerial/x2D setup with solid power now — will report back on whether that resolves the EAGAIN registration failure too.

Thanks both for the help.

You guys rock!

Confirmed — the weak 5V supply was the entire problem. With a proper 5V/7A supply in place, the exact same x2D bitfile that failed with EAGAIN yesterday now registers cleanly and shows full SSerial field I/O (input-00 through input-31, output-00 through output-15, spindle pins, all live). Thanks again for pointing at power — solved it completely.