Need help with a model M mod

[monsterrideop]

Hello all. I have recently acquired, for free no less, a Model M that was about to be thrown out at a grade school. It is a Lexmark 82G2383 that looked to be in good condition and I thought that I could possibly make it work for my own use.

I looked at many different websites and decided to make use of a "Blue Cube" PS/2 to USB adapter and use it to replace the original PS/2 cord which was frayed beyond use.
I thought I could simply follow the directions that were posted here: http://zevv.nl/play/misc/ibm-usb/ but found that my PCB was of a different type than the one shown.


Board after starting the mod.

After some measurements I found that I would not be able to mount the adapter within the case like the site mentioned. So I came up with my own plan. By using four of the 8 wires within a CAT5, I could have a pigtail like connector sticking out the rear of the keyboard. It would be a bit awkward but it should work as long as the keyboard doesn't move around much.

I started by removing the connector for the original cable. I then tried to solder the four chosen wires, the solid ones to be exact, into the holes left behind by the connector. This did not work well do to the old contacts lifting off of the PCB. After removing the failed attempt I got around to following the traces and soldered the wires to points on the top of the board where they would, at least electrically, be on the same trace.

I then worked on removing the PS/2 connector from the blue cube and was about to solder on the wires when I realized that I had no idea what wire was for what portion of the PS/2 connection. I started following the traces again and quickly discovered the VCC and GND wires. I then had only two left. They both lead into, what I believe to be, the PS/2 interface IC. However I have no idea which one is the data and which is the clock.


The IC in question.

I cannot find any information on this IC. I can tell you that the Brown wire is connected to pins 5 and 12, and the Green wire is connected to pins 2 and 3. Any help on which is the clock and which is the data would be very helpful.

[keyb_gr]

Looking at various M control PCBs, U1 sometimes was a SN7406 (hex buffer). Check whether that makes any sense.

Green wire (going to two pins) should be data, that needs to be bidi after all.

[monsterrideop]

I would like to say it makes sense but if it is a hex buffer than why are only 4 lines from U2 connected to the buffers leading to the PS/2 lines?
That and the PCB PN doesn't match up with the one shown on the site. Although that doesn't necessarily mean anything.
Oh, I also found out that the brown wire also goes to two pins on U1. I've corrected my initial post to reflect this.

[monsterrideop]

OK, so I did some more work, followed the traces, and came up with this:


This is a basic drawing of where the traces from each pin leads to and what they would map to assuming that this is a SN7406. If this assumption plays out, as it looks like it might, then I would cause a guess that PS2a(aka the brown wire) is the clock and that PS2b(aka the green wire) is for data.

This assumption would be based on the fact that the PS2a connection is being inverted before being passed onto the micro-controllers interrupt pin and that the PS2b connection is set up to be connected to two data pins on the micro-controller in a bi-directional manner.

If anyone comes up with a better, and more logical, explanation than please hash it out for me. Oh and ignore my previous post as this seems to supersede it.

[monsterrideop]

I noticed that one and considered doing it so that I would have a better looking connection. The problem lay in the fact that my PCB is not the same in that it does not have an SDL connector nor those two extra holes that are used with the bolts. I would have had to add the two holes and would still need to figure out the clock and data wires.
It is a good mod though. If I end up with another model m in my possession I'll probably end up following it instead of doing my own thing.

[Sam]

Yeah, that chip you couldn't find any information on is almost certainly just a 7406.  Here's a schematic of a Model M controller board:
http://www.kbdbabel.org/schematic/kbdbabel_doc_ibm1394796.pdf

I believe the SDL connector pinouts are:
1: Ground (Reserved on some boards)
2: Data
3: Ground
4: Clock
5: +5Vdc
6: Reserved

I'd say try to follow the traces on your board to see if they match that schematic or if you have a multimeter, check it out with that.  If the schematic matches, you should know which one is clock and which one is data, provided my information on the SDL connector is correct.

Personally, if I was doing it, I'd probably just randomly choose one line as clock and the other as data and try it.  If it didn't work, then swap the lines.  They're both the same levels and I've never blown anything doing that sort of thing.  However, I don't recommend other people do what I do, being I don't want to end up recommending something that just might possibly end up frying something.

Just had a look at Sandy55's page and it looks like my information on the SDL connector is wrong.  Swap clock and data.  #2 is Clock.  #4 is Data.

http://sandy55.fc2web.com/keyboard/model_m_trivia.html

[monsterrideop]

Got it. Turns out either my presumption was wrong, or I followed the traces incorrectly. My brown wire turned out to be the data and the green wire turned out to be the clock. I had hooked them up backwards and the leds started blinking and then suddenly stopped and none of the keys worked. Then I switched the wires and bang, one working keyboard, verified by the fact that I am using it right now.

Thanks for the help everyone, especially to Sam for the schematic.