Reply 720 of 805, by Arkku
Is it the same with another unused IRQ? Did you check IRQ3 that we know for sure works?
You don't have a scsi card installed or a network adapter, right?
There are no other cards installed at all, i.e., it's either the MPU or no cards. There is also no SCSI or network integrated on the motherboard.
IRQ 3 (and indeed all other IRQ pins) show "high" on the logic probe during power-up. Curiously, so does the serial port's IRQ 4 pin. I'm not entirely familiar with how exactly these IRQ pins work on an electrical level, so maybe this is normal. Anyway, the IRQ 3 is indeed known working and on an electrical level at the ISA connector looks no different from IRQ 2.
edit: Another thing I tried was installing a 3COM Etherlink III card instead of the MPU. It has software-configurable IRQ, so I tried IRQ 9 on it to see if that works. It doesn't, but again works on other free IRQs. So I think we have to conclude that IRQ 2/9 is just not being delivered correctly on this system. An extreme solution would be to physically trace the line all the way on the motherboard, but it may be quite difficult to do so, and I suspect in the end it will just go to some unknown pin on a large IC.