Thanks for the responses.
All of them point the finger at the use of flat ribbon cable vs "proper" GPIB shielded cables, and I agree, this is the very obvious suspect.
But my gut feeling is that this is not the problem.
@coromonadalix,
Using a separate interface for the M107 may end up being the solution, but I'd like to avoid that.
As to ground loops and control voltages on the pins, isn't this the exact same wiring (i.e. DC wise) as when proper cables are used?
A proper cable does have shielding, and uses twisted pairs. (at least my cable has alternating grounds for the for the handshake signals)
Not that there aren't other considerations associated with shielding, but the only reasons I've seen in documentation by HP and others for shielding is to reduce EMI, rather than to ensure proper operation.
And after all, I'm only looking at a bus length of 2 meters maximum, nowhere near the 20 meter max specified for GPIB, and with at most 5 devices on the bus, being well below the limit of 15.
If the lack of shielding etc. was the cause of the problem, I'd expect to sometimes see more general "flakey" behavior, but this M107 reset issue is the only incorrect behavior I've observed. All other commands work correctly every time on all 3 devices.
The reset problem has never happened with DMM#2, but happens with DMM#1 every single time.
@Gribo,
I've tried using just the M107 and DMM#1, using only two cables of the shortest possible length (30cm), and also tested with the device positions on the cable swapped both ways - still always the same behavior.
I can't help thinking of DEC PDP-11 computers that allowed the Unibus to extend for 10+ feet, also using flat ribbon cable with alternating ground wires, and it still transferred data at megabytes per second. This was also an active-low, open collector, handshaking system. It did use special drivers and receivers, but standard TTL has been used without issues.
@Kean, The offer to let me try proper cables is most kind, and I may someday want to take you up on that. Are you in Melbourne by any chance?
My next approach might be to see if I can swap parts of the HPIB sections between the two HP3456A's, and see where that leads me.