A little more on the HP120 keyaord interface

[Tony Duell]

As I mentioned in another thread, I spent a little time inside my HP120 
today. I've had another look at the keyboard interface.

Of the 6 pins on the keyboard connector :
1 is not conencted
2 are +12V and ground
2 are outputs (machine to keybaord)
and 1 is an inputer (keyoard to machine)

The 2 outputs are a scan clock (pulse to move to the next key in the 
matrix) and reset (start at the first key again). The input is, of 
course, to indicate if the selected key is pressed. These logic signals 
operate ate 12V levels

It is electrically indentical to the HP150 (origianl, not 150-II) 
keyboard interface. To the extent you cao plug an HP150 keyboard into an 
Hp120 without damage (I have done this after some careful checking). It 
doesn't work properly because the keyboard matrix is totally differnt (so 
keys to not produce the characters marked on them, and worse still, some 
keys don't exist), but it does something.

The HP150 keyboard electroncis is quite simple. It consists of 5 
4000-seires CMOS chips. A 4024 counter is driven by the 2 outputs from 
the computer (via diode/resistor protection networks). A couple of 4028 
decoders, driven by the counter, with a 4011 gate to inver the D input of 
one of them, scan the columns of the switch matrix. A 4051 mux, also 
driven by the counter, cans the rows. And the output of that is is 
buffered by a couple more 4011 gates and sent back to the computer.

By shorting outputs of the decoders to the inputs of the mux, I managed 
to work out most of the HP120's keyboard matrix. 

I how have a choice. I either copy the HP150 scan circuit (described 
above) -- the chips are trivial to get -- and wire up a matrix of 
switches. Or I make a little interface (presumaly using a 
microcontroller) to like a PC keyboard to the HP120. 

-tony