dionb wrote on 2024-01-29, 19:37:
Nope. In fact behaviour with one installed can be downright weird. I was troubleshooting a card that stopped an XT system from booting (probably a resource conflict, but could also be a dead 74-logic chip somewhere) and decided to stick my (16b ISA) POST card in on the off chance it told me something. It didn't - but with the card in the bus the system would boot despite the presence of the problematic card. If I removed it, system failed to boot again so long as the card was in there... I'm sure this is trying to tell me something fundamental, but for now I'm just scratching my head at the weirdness.
A POST card is meant to be "read only" on the ISA bus, so in theory the presence of the POST card is not supposed to change the behaviour of the system. A POST card is not expected to add wait-states to any cycle or to respond any kind of read. In practice every card on the the ISA bus provides some capacitive load, and (likely more importart) works similar to a pull-up resistor if actual TTL-type (in contrast to CMOS-type) inputs are used. For a period-correct 16b ISA POST card, having TTL chips on it is quite likely (most likely 74LSxxx stuff).
So I guess your POST card changes the behaviour of the system because it pulls a lot of lines on the ISA bus to a slightly higher level, especially when they are not driven by anything. There are a lot of different possible root causes for your symptoms, though
- A bus driver on the mainboard might be partially blown and unable to pull an active low control line high enough if that line is supposed to be inactive.
- A receiver on your problematic card is broken and requires a higher level (~3.5V) instead of the standard 2V to correctly recognize a high level.
- You actually do have a resource conflict with two cards fighting each other, and the extra pull-up power changes a bit that is driven low and high at the same time to "high".
- It's acutally not the pull-up action, but the capacitance smoothing out some edges and damping ringing.
I'm sorry I can't tell you what kind of these "fundamental" (to quote your words) things the system tries to tell you, though.