Making an IBM M4-1/spacesaver II cable

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06 Feb 2020, 01:39


I've recently bought a new old stock Lexmark M4-1. It was horrible but after swapping the rubber sleeves with one from an IBM L40SX keyboard it types pretty good.

It came with a sealed manual but not the SDL to 2× PS/2 cable for some reason.

After checking out this this thread it seems that Unicomp actually still sells this cable new. It multiplexes the input from the Trackpoint II along with the keyboard input and sends it over two male PS/2 plugs.

Rather than ordering a new cable, I want to make one myself by modifying an existing SDL to PS/2 cable. The Y cable looks completely passive and must be simple to make by soldering another PS/2 cable on to it, right?

If something like this is possible and done before, please let me know. Even a picture of how the Y connection is wired would be great.


06 Feb 2020, 02:15

Yes, the middle 4 pins cary the keyboard signal and the outer two must cary the mouse signals. A regular SDL cable plugged into a Trackpoint keyboard will just ignore the mouse. I'd just take the pinouts from a regular SDL cable and if you get the outer pins reversed, it shouldn't cause any damage. The only thing that the "Y" likely splits is the GND and +5V as those are common to both the mouse and the keyboard.

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13 Feb 2020, 00:32

It turns out the two outer pins (SDL pin A and F, which are PS/2 pin 2,6 and labelled "reserved") are missing entirely from my SDL cable, which makes sense as to it would be wasteful to implement the wires for them. I guess this means it is not doable by modifying an existing cable unless if I where to entirely build my own new cable.

So, its not doable

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