Very interesting post from a similar thread. I think I'll start by replacing the keyboard controller.

canthearu wrote on 2018-12-23, 08:04:

Yep, at least one of the OPs faults is centered around the fact that the chipset isn't able to control the A20 line. The A20 lin […]
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Yep, at least one of the OPs faults is centered around the fact that the chipset isn't able to control the A20 line. The A20 line does indeed get controlled by the keyboard controller in 286 based systems.

The A20 line is the address line that lets the chipset choose between the first megabyte of system ram and the second megabyte. On old XT systems, the first megabyte of RAM is all there was (no A20 ), so if you tried to access memory FFFF;0010, you would actually access the very first byte of memory, location 0. In other words, with the segment system, memory would wrap around.

The 286 processor however, if you try to access FFFF:0010, would try to access the memory at location 100000, rather than 000000. To maintain compatibility with that old wraparound the XT had, IBM routed A20 through the keyboard controller and then you would program the keyboard controller to either block A20 or not. If you block reaching it the memory banks, so the memory always saw A20 as 0 on the address bus, the old XT behavior occurred when accessing FFFF:0010. If you enable normal A20 use, then FFFF:0010 would access memory location 10000. This is how MSDOS can load a portion of itself high, by using the memory space that would normally wrap around to 0 on an XT.

To do the memory test, A20 has to be enabled so when the 286 switches to protected mode, it can access all the memory. If it can't enable A20, the system board tends to panic and do what the OP sees. The virtual mode memory test probably refers to the fact that the CPU is being placed into protected mode to do the memory test, which is what one would expect.

So yes, definitely check over the keyboard controller, it's solder points along with all nearby traces, the chipset as well. there is probably just a bad connection somewhere causing this, as the system is so close to functioning at this point.

Re: MAT286 problems / won't start / Gate A20 error - please help with diagnosis

No luck with this board yet.

I have confirmed that the companion board aka "personality board" is good, and can be used as an EGA card paired with a different SBC.

Back to the main board though. I got my POST card a while back. Initially it was showing code 08, which looks consistent with the RAM error posted earlier. Noted the lack of CLK indication too.

The RAM is soldered, so I put that investigation on the backburner and went for some low hanging fruit first. With some kind help from a TL866 owner I got the BIOS chips reflashed. All seemed fine with these (before & after) so they were ruled out as the problem. In the meantime I've also picked up a T48 for myself. Although probably futile at this point, I did flash another pair of chips which offered no improvement to the situation.

I've also tried a bunch of different KBCs. I even picked up a few of the exact same type. I know this is a complete crapshoot because the programming needs to match the board - but it was a cheap hail mary anyway. Maybe I'd get very lucky! I did not.

But there has been some progress. The POST card now only displays "--". It's more lifeless than ever.

Now I want to look into that missing CLK signal. I've ordered a cheap logic probe, even though I'm not quite sure where to start with it when I get it. Any advice appreciated!

Pierre32 wrote on 2022-08-21, 00:09:

But there has been some progress. The POST card now only displays "--". It's more lifeless than ever.

Correction. The board always booted to "--" most of the time, only occasionally throwing a code. I had kind of forgotten that in today's testing. So it hasn't actually worsened. Today I tried a million or so boots, and when the card responded it showed me 0C:

Which makes me start looking at that KBC again...

you sure its award bios? afaik award never had such elaborate diagnostic screen

Any chance of getting a bios dump ?

I had around 3 of these boards and none of them was working correctly. They powered up maybe every 20th time. Seems not to be the best quality.

80286 BIOS image collection

mmmh

bad solderjoint(s) ? faulty contacts on the ic sockets ? marginal chips due to age/exposure ?

funny to see the difference in bodge wires

Predator99, Couldn't you have said something sooner! you could've saved Pierre32 a few months of troubleshooting 😉 (jk)

Pierre32, good luck chasing the rabbit.

The "battery power lost - run setup" message is irritating. Where´s that battery?

Is this onboard ram 128K? Looked like 64K x 1 chips?

Cheers,

Cheers for all the replies!

rasz_pl wrote on 2022-08-21, 12:37:

you sure its award bios? afaik award never had such elaborate diagnostic screen

Hmm, trying to remember now how I decided that. Here are some Award screens that look similar (but not quite the same):

https://i.ytimg.com/vi/bCYKPmk08e8/maxresdefault.jpg
https://www.eevblog.com/forum/repair/help-tro … daku4necmqb81k4

weedeewee wrote on 2022-08-21, 14:12:

Any chance of getting a bios dump ?

Attached.

majestyk wrote on 2022-08-21, 19:46:

The "battery power lost - run setup" message is irritating. Where´s that battery?

No battery. The connector is at top right, obscured by the RAM daughterboard. FWIW the blue bodge wire heading down from the top connects one of the battery pins to the bottom of the chip near the edge connector. I know many old boards simply won't POST without a battery present, however since this one has done everything described in the thread without a battery, I haven't considered that to be the issue here.

pentiumspeed wrote on 2022-08-21, 21:05:

Is this onboard ram 128K? Looked like 64K x 1 chips?

Cheers,

From the manual:

"Also on the board arc two banks of dynamic RAM. Each bank consists
of nine chips; eight chips are for data and the ninth is for parity. These
chips provide 512KB of base memory. RAM memory may be increased
128KB for a maximum of 640KB base memory."

The expansion daugherboard is present, so this system should have 640KB.

Pierre32 wrote on 2022-08-21, 22:26:

The expansion daugherboard is present, so this system should have 640KB.

And it shows 256KB?
Broken RAM or misconfigured?

Most likely a misconfiguration at the time. There is a jumper (JP3) that selects between 256K or 512K onboard.

(Edit- And of course a jumper that enables the additional 128K board).

Predator99 wrote on 2022-08-21, 18:42:

I had around 3 of these boards and none of them was working correctly. They powered up maybe every 20th time. Seems not to be the best quality.

80286 BIOS image collection

Deciding how much energy I should pour into this thing, and this post has been ringing in my ears. Multiple other examples behaving the same way as mine doesn't bode well.

Did you put much time into troubleshooting your boards? What kind of things did you try?