I found this document for the ALI chipset. So it seems the BIOS is not on the ISA bus which is weird as it goes onto the ISA bus via a LS245.

Anyways, the BIOS communicates with the CPU via a bus which goes to the Southbridge which goes to the PCI bus which goes to the Northbridge which goes to the CPU. Complicated.

Can anybody think of a way to remove some variables here?

tony359 wrote on 2024-02-26, 17:21:

Hi all, ... I would like to understand better what happens when a system is powered up. ... Thanks for now! […]
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Hi all,
...
I would like to understand better what happens when a system is powered up.
...
Thanks for now!

Hi!
I think you know about a thing called "Power-On Sequence diagram". Though I really doubt that there're ones for vintage motherboards.
IMO, a datasheet of chipset is a good starting point to fully understand how a system works. Of course it's a hard way. I hope the schematics will help a little bit

This really helps, thank you! I guess the V58 won't be so different at least on the basics!

I've now probed things on the CPU end (at the back of the CPU) and I do not see any activity whatsoever. Nothing.

There is clock, voltage and reset works. But the CPU does not start. I checked the HOLD, HLDA (Hold Acknowledge), STPCLCK pins, nothing points to something requesting the CPU to stop.

Are there other pins which might be preventing the CPU from starting? I do feel it's the motherboard somehow (a jumper?) to tell the CPU not to start. My guts feelings 😀

tony359 wrote on 2024-02-27, 12:17:

There is clock, voltage and reset works. But the CPU does not start.

Do you mean CPURST signal is 0 (inactive) ?

Can I check which one is CPURST? I have a RESET on the pinout, AK20 - that goes high for a moment and then it goes low as expected.

tony359 wrote on 2024-02-27, 19:11:

Can I check which one is CPURST? I have a RESET on the pinout, AK20 - that goes high for a moment and then it goes low as expected.

Then it's OK. Also I would inspect for scratches places near board's mounting holes and socket7 CPU-FAN hooks.
Once I repaired a mainboard that behaved like this (turned on but didn't post) because of just one broken wire near a CPU socket.

Unfortunately I've inspected the board physically multiple times, it's in very good nic and I cannot find any damage. Hence why I started probing pins! 😀
My guts feeling say that it's the motherboard itself stopping the CPU. When I got it, the RESET line would not disable and turned out there is a "Resume Power Supply" feature which can be disabled. I suppose the board supports some sort of pre-ATX power supplies - it also has a Power On pin header for a switch.

Once the "Resume PSU" is set to "Traditional", the reset line works. But is still doesn't do anything. I have a feeling there is some other jumper which might be causing this. The manual is only 90% accurate, some jumpers are mentioned incorrectly. There is also a "HW Monitor" feature which is barely mentioned in the manual.