appiah4 wrote on 2024-01-29, 14:43:

I have been offered an 8088 board that is apparently not working. I have questions..

1. Do all 8088 motherboards need XT PSUs? Are there XT boards made to the AT standard?
2. Do I need an XT keyboard to test the board? Do AT to XT adapters exist for keyboards?

1. XT power supplies have a built-in button and a different form-factor, but their pinout is the same as AT. So you absolutely can use AT PSU unmodified with XT as long as it's a standard (non-proprietary) board. See here.
2. Yes, you do. There are AT/XT keyboards with a switch, there are also AT 2 XT and even PS2 2 XT adapters. I bought mine from some local tinkerer a long time ago, pretty sure there is an open source design everyone reproduces. Just google for AT2XT.

Some clone XT boards have square pin power connectors, which are still same XT/AT layout, you can usually force a normal connector onto them though.

edit: standard-ish looking boards tend to be a bit larger than Baby AT format footprint, cases that only take baby AT like mini towers, might not have quite enough space to fit one. The holes might only almost line up too, take care there's no built in standoffs or screwin brass standoffs that will short.

jheronimus wrote on 2024-01-29, 15:11:

appiah4 wrote on 2024-01-29, 14:43:

2. Do I need an XT keyboard to test the board? Do AT to XT adapters exist for keyboards?

2. Yes, you do. There are AT/XT keyboards with a switch, there are also AT 2 XT and even PS2 2 XT adapters. I bought mine from some local tinkerer a long time ago, pretty sure there is an open source design everyone reproduces. Just google for AT2XT.

There also are keyboards that don't have a physical switch, but can auto-detect whether they are connected to an AT or an XT, and adapt appropriately. In case of an untested board, I'd suggest to use a keyboard that is known to support the XT protocol, though.

A generic Turbo XT board will tell you during POST if the keyboard isn't compatible. In general, as with later systems a speaker can be useful to diagnose boot issues, as if the BIOS is able to initialize at all it will loudly complain if not have.

dionb wrote on 2024-01-29, 16:58:

A generic Turbo XT board will tell you during POST if the keyboard isn't compatible. In general, as with later systems a speaker can be useful to diagnose boot issues, as if the BIOS is able to initialize at all it will loudly complain if not have.

Adding a speaker to get beeps is a good idea. On the other hand, don't expect to get POST codes on a Port-80 diagnostic card. Most XT BIOSes do not use Port 80 for diagnostic codes.

mkarcher wrote on 2024-01-29, 17:39:

dionb wrote on 2024-01-29, 16:58:

A generic Turbo XT board will tell you during POST if the keyboard isn't compatible. In general, as with later systems a speaker can be useful to diagnose boot issues, as if the BIOS is able to initialize at all it will loudly complain if not have.

Adding a speaker to get beeps is a good idea. On the other hand, don't expect to get POST codes on a Port-80 diagnostic card. Most XT BIOSes do not use Port 80 for diagnostic codes.

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.

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.