Now, let's access those configuration registers

User manual for the HP E1326B and HP E1411B multimeters

Annoyingly enough, it returns the register contents as signed decimals. Ugh

And there is no way to change that.

Poking around in software I can't find a reason why Error 13 occurs. There's got to be a hardware difference between the VXI mainframe and this plain VME backplane, despite the standard saying that VXI is a superset of VME.

So let's look at the VXI frame again.

There's a little board in the VXI frame that supplies the ACFAIL* signal. Just two LM339 quad comparators. Disconnecting it does not result in the same behavior as the plain VME backplane.

Another look at the VXI backplane. What I presumed to be simple buffers are actually 74ACT32. Quad 2-Input OR gates. Maybe it is some bus arbitration issue? Maybe they are just used as buffers?

I cannot find a schematic for the backplane, of course not. And I couldn't find anything in the standard either.

I can buzz out the upper row without having to take the chassis apart.
First column are the OR-gate's pins, second the functions, third what they connect to. BGnIN* and BGnOUT* n = 1..4 are bus grant lines needed for busmastering. Those gates are not present on my plain VME backplane. But I have my doubts that this could be the reason. I doubt my meters even use busmastering. I scoped one of the lines out and couldn't see any activity.
Guess I have to take the chassis apart and maybe even go in there with a logic analyzer to find out what is causing Error 13.

And at this point I decided to give the PSU in the mainframe another chance. Turns out, it is fine as long as I keep that 200dB 240V mains powered fan connected. My poor ears

I will order replacement fans for this and have another look at the PSU again. But not now. Onward! No more rabbit holes.

I wasn't able to get a response from the command module on GPIB before. I found out why. My python setup is not working in the first place. Dug out some other piece of GPIB test gear I know works to figure that out.

But I also need to address the command module using a non zero secondary address (sad). You'd expect the CM to live at sad=0 and primary address (pad) whatever you set it to, in this case pad=9. But there is silence. You need to add 96 to the sad, at least for linux-gpib. What an odd quirk!

It lives! 🎉