VOGONS


First post, by dylanfm

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I'm trying to fix an old Model 25 and I'm getting a 301 keyboard error and press F1 to continue while booting. I'0ve tried the 2 keyboards I have that have a PS2 type connector and both give the same error. I also checked the fuse next to the keyboard connector and it is good. Is there something special about the keyboard I need? Is a common error? Any ideas about what I should do to fix this?

Full history as I know it (since I only got this computer yesterday):

When I got it, there was a burnt up cap taped to the top of the case. When I tried to turn it on the first time, the power light would come on for a second and then turn off. When I inspected the board I found that several of the caps have already been replaced. Eventually I found that cap on the -12V rail was shorted. Since I don't have a replacement and I wanted to see if that was the problem, I just cut it out. I also noticed that another capacitor on the +12V rail had been cut out and was missing (presumably the one taped to the outside of the case). After that, when I powered it up the floppy drive let a lot of smoke out. I read somewhere that these floppy drives are prone to needing all the caps replaced. So I removed the floppy drive. Now when I power it up I get a floppy drive error and then the keyboard error I mentioned above. I don't think not having those 2 bypass caps on the +12V and -12V rails would cause the keyboard not to work, but maybe...

Dylan

Reply 1 of 3, by mkarcher

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dylanfm wrote on Today, 01:13:

Is there something special about the keyboard I need? Is a common error? Any ideas about what I should do to fix this?

I got a Model 30, which should be close enough in that regard. I typically use it with a IBM Model M keyboard, which has an AT-style physical interface, but it indeed is special: AT keyboards are supposed to have three modes of operation:

  1. The keyboard sends XT-compatible scan codes.
  2. The keyboard sends AT scan codes that can be 1:1 mapped to XT scan codes and uses magic numbers to signify any extended keys like the dedicated cursor keys.
  3. The keyboard sends native extended scan codes, in which the keys all just have a simple number and no magic is required.

The IBM AT and all AT compatibles use the keyboard in the second mode and have the keyboard controller translate the AT scan codes into XT scan codes, which are then handled by the BIOS or DOS applications that perform raw keyboard access. The IBM PS/2 Models 25 and 30 do not have a keyboard controller, and they do not implement this translation in their custom preprocessor on INT 71. Instead the BIOS sets up the keyboard to operate in the first mode instead. In case of late 90s standard OEM keyboard, it is possible that this mode is not properly implemented. On a related note: Different operating systems tried to use mode 3 because it simplifies keyboard handling, and they all backed out to use mode 2 by default (or just removed mode 3 support entirely) because of compatibility issues. So even a seldomly used mode that is actually useful is not compatible with all PS/2-style keyboards, so I wouldn't be surprised if mode 1, which is most likely exclusively used for the low-end PS/2 models is missing from a lot of "modern" PS/2 keyboards.

dylanfm wrote on Today, 01:13:

After that, when I powered it up the floppy drive let a lot of smoke out. I read somewhere that these floppy drives are prone to needing all the caps replaced.

Yeah, those drives are infamous for dying caps. The one that emitted a burning smell on me, though, didn't have a burning cap, though, but a shorted cap likely blew an interference suppression inductor near the drive connector. So I suggest if you recap the drive, you check those inductors for continuity, too.

Reply 2 of 3, by dylanfm

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Thanks for the info.! It sounds like I need to find a Model M keyboard because it doesn't seem like there is a good way of knowing if they are compatible with the Model 25. Or at least I'm not finding that info with Google.

Reply 3 of 3, by luckybob

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I dont think i've ever had a PS/2 keyboard that didnt work like MKARCHER described. Anecdotal evidence is anecdotal though.

Can you try the other PS/2 port? it /SHOULEDN'T/ matter, both ports should work with mouse/kb. that said, it sounds like this board has some damage that needs to be addressed before going forward. Any chance we can get a picture?

BTW the model 30 is the same motherboard as the model 25. 😀

Once men turned their thinking over to machines in the hope that this would set them free. But that only permitted other men with machines to enslave them. - Reverend Mother Gaius Helen Mohiam