Keyboard adapter for the HP 200 series of microcomputers

What if you could use a standard USB-keyboard with your old HP 200-series computer? A HP9816, HP9826 or perhaps a HP9836? Why would you want to do that? Well, the keyboard might be missing or broken, or simply have the wrong layout. This small contraption allows you to use a standard USB keyboard and select from the supported international layouts of the HP98x6 to boot!

Click on the schematic for a larger PDF

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The 98x6 keyboard protocol

The keyboard protocol is a simple bidirectional 2-wire protocol. One clock and one data. The data-line is an open-collector interface. The host starts a poll sequence by pulling the data line low and then starts outputting clock pulses.

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Every key has a corresponding number and when say key 10 is pressed, the data line is pulled low after 10 clock pulses.

At the end of the scan sequence, there is a cluster of pulses. They are 8 pulses from the encoder and 8 pulses from the configuration links on the keyboard. These are used to select keyboard layout (language). There is no documentation for these, other than Tony Duell's schematics, so I had to reverse-engineer them. If we see the combined two as a 16-bit integer, then we get:

ValueExplanationSwitch
0b1111111111111111US0
0b0001111111111111Swedish/Finnish1
0b0011111111111111French?2
0b0101111111111111German3
0b0110111111111111Spanish4

Disclaimer

This information and the circuits are provided as is without any express or implied warranties. While every effort has been taken to ensure the accuracy of the information contained in this text, the authors/maintainers/contributors assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. I disclaim everything. The contents of the articles below might be totally inaccurate, inappropriate, or misguided. There is no guarantee as to the suitability of said circuits and information for any purpose whatsoever other than as a self-training aid. I.E. If it blows your equipments, trashes your hard disc, wipes your backup, burns your building down or just plain don't work, IT ISN'T MY FAULT. In the event of judicial ruling to the contrary, any liability shall be limited to the sum charged on you by us for the aforementioned document or nothing, whichever is the lower. I will not be held responsible for any damages or costs which might occur as a result of my advice or designs. Nor are you allowed to use any of my designs for commercial purposes without my written authorisation.

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