Later model turbo XT clone motherboard has weird issues

[Mike Chambers]

I've got a 10 MHz XT clone board with a V20, not sure of the brand. It's a later model that seems to be more integrated than most, and it's not very large.

Anyway, lately it's got a strange issue where it always throws a KB error on post, yet the keyboard works fine. It happens with any keyboard I connect.

I also recently booted it up (XT-IDE + CF card), loaded a packet driver and tried to run a program. It just hangs. I've tried multiple NICs, not sure if it's even related to the networking or not. The KB error makes me feel like there's a more general issue somewhere with part of the chipset. The same program works fine on any other XT-style system.

This is a clean MS-DOS 6.22 boot, F5 to load without config.sys/autoexec.

Any thoughts on this one?

EDIT: Well the program lockup turned out to be a silly issue. The CF card is formatted with a different CHS layout than what the XT-IDE is picking it up as, so some things will read the wrong sectors when loading!! At least MSD reports something different than the image I flashed onto it, so I'll see if that resolves it.

Still bugged by the keyboard error on POST though.

 

[kdr]

I'm assuming that the keyboards you have tried are known to work with other XT systems.

Perhaps you could check that the RESET and CLK lines to the keyboard are working correctly. If the keyboard isn't getting reset properly at power on that might cause it to fail POST despite working normally after boot. Does the keyboard still cause a POST error during warm boot after hitting Ctrl-Alt-Del?

 

[modem7]

Some possibilities:

1. The particular motherboard has a keyboard fuse, which is blown. (Results in no +5V supplied to keyboard.)
2. Cracked solder joint/s where DIN socket is soldered to motherboard. (Cracked due to mechanical stress of repeated plugging/unplugging of keyboard.)
3. Motherboard was exposed to a leaking NiCad battery, and the leakage opened up one or more PCB tracks.
4. Faulty keyboard interface circuitry (chips and PCB traces) on motherboard. Will be similar to [here], although most of that may be within a chip set.

Regarding point 1. With the keyboard removed, see if there is +5V measured between the ground and +5V contacts of the motherboard's DIN socket. Using the ground contact in the socket, rather than a ground elsewhere, also confirms continuity of ground through to the ground contact of the socket.

Whilst you are doing that, see if +5V is present on the DATA and CLOCK contacts. We expect +5V on those contacts due to pull-up resistors on the motherboard, together with the motherboard not driving those contacts LOW (at this time).

 

[Mike Chambers]

I'm assuming that the keyboards you have tried are known to work with other XT systems.

Perhaps you could check that the RESET and CLK lines to the keyboard are working correctly. If the keyboard isn't getting reset properly at power on that might cause it to fail POST despite working normally after boot. Does the keyboard still cause a POST error during warm boot after hitting Ctrl-Alt-Del?

Unfortunately, I don't have a scope so I don't know if I can go as far as checking the CLK line. Though maybe I could rig something up to check for pulses on an Arduino. It doesn't cause an error after a warm boot. And yes, both keyboards I tested work perfectly on other boards.

Some possibilities:

1. The particular motherboard has a keyboard fuse, which is blown. (Results in no +5V supplied to keyboard.)
2. Cracked solder joint/s where DIN socket is soldered to motherboard. (Cracked due to mechanical stress of repeated plugging/unplugging of keyboard.)
3. Motherboard was exposed to a leaking NiCad battery, and the leakage opened up one or more PCB tracks.
4. Faulty keyboard interface circuitry (chips and PCB traces) on motherboard. Will be similar to [here], although most of that may be within a chip set.

Regarding point 1. With the keyboard removed, see if there is +5V measured between the ground and +5V contacts of the motherboard's DIN socket. Using the ground contact in the socket, rather than a ground elsewhere, also confirms continuity of ground through to the ground contact of the socket.

Whilst you are doing that, see if +5V is present on the DATA and CLOCK contacts. We expect +5V on those contacts due to pull-up resistors on the motherboard, together with the motherboard not driving those contacts LOW (at this time).

The fuse is good, I get 4.93 volts from it, and I read 9.3 ohms across it. Solder joints look excellent. Traces also look great to my eye. In fact, visually, this whole motherboard looks just about mint.

There's no battery on this. No RTC or CMOS to save.

However, after writing this post up, I was looking at the board. I saw a jumper that switches between 27128 and 2764, which sound like ROM types. I switched the jumper position expecting the PC to not boot but just thought I'd try random stuff. It worked.

The message is gone!

 

[kdr]

However, after writing this post up, I was looking at the board. I saw a jumper that switches between 27128 and 2764, which sound like ROM types. I switched the jumper position expecting the PC to not boot but just thought I'd try random stuff. It worked.

The message is gone!

That's rather interesting behaviour... hmm.

At least it wasn't a hard-to-track-down hardware fault on the board!