HP 700/32 keyboard signals

[Tony Duell]

> 
> Hi,
> 
> An acquaintance in (maybe) India, has dozens of HP 700/32
> keyboards he'd like to continue using ... but by attaching them to

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...

> modern equipment.  He's done some initial work trying to figure
> out the signals/protocol, but hasn't solved it yet.
> 
> I pointed him to CC here, but he seems reluctant to join.
> 
> He writes:
>   > We have about 75 HP 700/32 terminal keyboards which we want use as input
>   > devices for our quality control points. I have already constructed a PIC    
>   > board to do the job but I just can not read the keyboard. What I see from
>   > the osciloscope is, the keyboard requires 5V power, a 77 kHz 1:3 duty rate
>   > clock, and outputs two pulses on two different lines, time between which is
>   > dependent on the key pressed. For example, for the left-shift key, there is
>   > always 22 clock cycles between these two pulses. 

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).

 
> If anyone is interested in corresponding with him, please email me offline
> and I'll try to put you in contact with him.

Feel free to forward the above, but I am not sure I have anything else I 
could say.

-tony