| 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
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.
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:
| Value | Explanation | Switch |
|---|---|---|
| 0b1111111111111111 | US | 0 |
| 0b0001111111111111 | Swedish/Finnish | 1 |
| 0b0011111111111111 | French? | 2 |
| 0b0101111111111111 | German | 3 |
| 0b0110111111111111 | Spanish | 4 |
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