IBM Model M - Need help troubleshooting ... (PC just keeps beeping)

sofakng

11 Jun 2021, 19:59

I've received a used IBM Model M (1391401) and I completely cleaned all of the keycaps and stems and made sure it was absolutely 100% dry.

When the PC boots, the NUMLOCK is correctly illuminated, but then the PC keeps beeping and displays Keyboard Error and nothing works on the keyboard.

I've cleaned the ribbon cables inside the keyboard (with isopropyl alcohol and Deoxit) but it didn't help. I'm not sure if the membrane is broken and I was hoping not to remove the rivets but I'm not sure if the problem is the membrane or the PCB.

Can anybody help me diagnose this?

User avatar
Tritian

11 Jun 2021, 20:43

are you directly connecting ps/2 to a ps/2 connector on the computer or are you going ps/2 to usb? if ps/2 to usb you may need an active converter instead of a cheaper passive one.

sofakng

11 Jun 2021, 21:13

I'm using PS/2 directly on an older Pentium PC.

User avatar
Weezer

12 Jun 2021, 10:08

Troubleshooting is all about identitying and isolating variables. Have you tried the keyboard on another computer? Have you tried it through a USB adapter? Have you done a continuity check on the cable?

The membrane and switches have nothing to do with a keyboard POST error. The connection or lack thereof between the keyboard controller and the i/o controller on the motherboard determines the POST outcome.

sofakng

12 Jun 2021, 16:33

Unfortunately I don't have another PC with a PS/2 port.

However, I have done some testing... If I unplug the 16-pin ribbon cable the beeping stops. (i.e. I can leave in the 8 pin and 4 pin cables)

User avatar
Weezer

12 Jun 2021, 21:55

I took a Model M apart and posted my computer before sending my message, and the computer was able to post with neither ribbon cable attached, or one attached, and I was able to hot plug both the cables back in and use the keyboard after I'd already booted into the OS.

If you're set on it being the membrane the only thing I can think of is that so many keys are erroneously registering as depressed that youre reaching your 2 key rollover lockout on every column.

Have you tried plugging the 16 contact ribbon cable back in after the computer posts?

sofakng

12 Jun 2021, 22:30

Yes, the computer POSTs without error if no ribbons are attached. It will also POST if the 8 pin and 4 pin are attached.

However, once I hotplug the 16 pin cable it begins to beep

User avatar
Weezer

12 Jun 2021, 23:53

Open a text file and then hot plug in the ribbon. Does it type a bunch of bs into the text file?

I think the beeping might just be your computer complaining that too many keys are being depressed.

sofakng

16 Jun 2021, 20:22

I've just tested this method and it didn't send any keystrokes/letters:

1) Booted to DOS and opened EDIT
2) Once it was open, I plugged in the Model M with 8 pin and 4 pin ribbons attached. No beeps.
3) Hotplugged 16-pin ribbon and beeping started but no text.

I've purchased some flat ribbon connectors to also try and troubleshoot this (and learn how keyboard matrices work)

Which ribbon cable is the columns and which are the rows?

Can I send 5v to each column individually and check for continuity with each row to see if any keys are being pressed?

Post Reply

Return to “Keyboards”