Problem with a MESA 7i76

There is no state change upon LinuxCNC startup. + outputs remain low, - outputs remain high. When I use PncConf>Test/Tune>Run for reciprocating travel, the +DIR output oscillates between about 50mV and 80mV, and the -DIR output between 4.915V and 4.920V during direction changes. So there is some signal getting through, it just doesn't appear to be getting amplified/conditioned appropriately?

6i25 firmware should be 7i76+7i85 (I was going to run glass scales, but didn't seem to end up needing to, but it works for a remote MPG box) and currently just running/connected the 7i76.

I'll continue poking around in software, maybe re-build the hal with a new PncConf runthrough and double-check some stuff in the terminal. Worked perfectly previously with no hardware changes, though.

OK I misunderstood. if the output pins do not change state on starting LinuxCNC its likely
that the 74ACT540 and 74ACT541 chips have been damaged (or the series resistor
networks on the outputs have been damaged)

This can happen in a couple of ways

1, Accidental contact of step/dir pins to 12/24V
2. accidental overvoltage on the 5V 7I76 supply (7V will destroy ACT parts)

Number 2 is worse since is will likely damage multiple 5V parts

Hmm... I don't recall anything happening during rewiring that would have caused either of those to take place. The only 5V in is via the parallel cable, all lights and 6i25 seem happy. I'm guessing though from the "74*" that those chips are some kind of shift register or the like, and the serial comms isn't going to notice that an open-loop output isn't working. Perhaps though, since I did see distinct signal patterns on the output, it's the resistors at fault rather than the ICs specifically.

Is there any reasonably doable method of diagnosis I can perform on my end to find out which is more likely? I see the S/D* and RN* screenprint labelling, but no obviously visible issues on the little 8-pin devices.

You can measure the resistance across the networks (if you have a steady hand)
they should be 27 Ohms

The 74ACT540 and 74ACT541 are just buffers

Appreciate all your rapid responses! Once I got going I figured I may as well do them all, though I doubt half of them matter to this issue. All individual resistors check out within a 5% tolerance of: RN1,4,18,25,26 @ 27R, RN11,13,20 @ 138R, RN10,12,19 @ 145R, RN3 @ 47R.

I guess it's the buffers. Not sure how, though, makes no sense. Is it feasible to test the buffers if I provide 5V bench power to TB3 and move the jumper? I do have a parallel terminal breakout lying around doing nothing if it helps to go in through P1 and the datasheet appears pretty straightforward.

Yes they are just basic non-inverting buffers (74ACT541 on + pins) and
inverting buffers (74ACT540 on - pins) connecting between the DB25 and
the 27 Ohm resistor networks