This is not a machine I have ever worked on, but I noticed to CCed the message to me personally, so I guess you think I might have some clues... A few obvious questions : Has he opnened up the keyboard, if so, what's inside (microcontroller, HP custom chip, SSI/MSI chips)? 2) How did he make that above measurents ? If it was by connecting a 'scope or logic analyser onto the cable between the terminal and the keyboard, how does he know which direction the signals are in. The point being there is a fairly well-known HP keyboard interface, used with modifications on several machines, which has the following 5 signals : Power (may be +5V or +12V) Ground Clock (but a lot slower than 77kHz normally) (from host to keyboard) Reset (from host to keyboard) Data (from keyboard to host). The basic protocol is that pulsing 'reset' clears an internal counter in the keyboard and selects the zeroth 'key'. Each pulse on 'clock' moves on to the next key in the sequest. And the data line indicates whether the currently-selected key is pressed or not. THis sounds remarkably like the signals he's observing (hence the delay between the 2 pulses being depecndant on which key is pressed). But normally only one signal is 'from' the keyboard. I do know that HP used a similar interface for the touchscreen unit in the HP150 (original version), but in that case the 'reset' (or 'sync') signal was output from the peripheral (touchscreen logic) and an input to the host. I have never come across this being used for a keyboard, but it's clearly possible. Other things. What happens if you hold down several keys at the same time (multiple pulses on the 'data' line)? Is the 'reset' / 'sync' pulse always present, even if no keys are pressed? What lines have signals on them if the terminal is powered up with no keyboard connected ? (this _might_ indicate the direction of the signals). Feel free to forward the above, but I am not sure I have anything else I could say. -tony