HP262x keyboard to HP120 interface
ard at p850ug1.demon.co.uk
Sat Apr 11 13:17:23 CDT 2009
For those who are still interested in this (Rik?), I've now built the
interface from an HP262x keybaord (actually one 'borrowed' from my HP2623
terminal) to my HP120.
The differenve between the 2 keyboard interfaces is quite simple. The
HP120 puts the scan counter in the keyboard, the interface being the
clock and reset lines for this counter, along with an active-high
'current key pressed' siganl. The HP262x puts the scan counter in the
terminal, the inteface is the 7 key select lines (basically the outputs
of the counter) and an active-low 'key pressed' signal (open-collector).
Although the Hp262x runs the keybaord at 5V, it's all 4000 series logic
interally, and can be run at the 12V of the HP120 interface.
So the interface between this keyoard and the HP120 is little more than a
7 it counter.
I've now built it, it's a little box with a DA15 socket on one end to
take the plug from the unmodified HP2623 keyboard (I obviously wanted to
leave the keyoard unchanged so I can still use it with the 2623 terminal)
and a short cable coming out of the other end ending in an RJ11 plug to
go into the HP120.
Inside are 3 chips, all common 4000 series CMOS parts. :
A 4024 (7 bit counter). This is linked to the clock and reset lines from
the HP120 via the resisotr/diode protection networks as used in an HP150
The bottom 6 outputs of this counter are buffred by a 4050 chip and then
fed via a 16 pin header plug nad socket to the DA15 socket for the
keyboard. This chip may not be necessary, but some counters don't like
driving long cables (glitches on the outputs can change the state of the
flip-flops). I dout the 4024 sufferes from this, but adding one chip to
be sure seems worth it.
The last output from that counter is buffered bu 2 sections of a 4049 in
cascade. The reason I used that chip is that I needed a NOT gate to
invert the open-collector key-pressed/ signal from the HP2623 keyboard.
This I did,, after pulling said signal high with a 3k3 resistor. The
output of that NOT gate is again given a diode/resistor protection
circuit and fed to the HP120
And it works. It worked first time actually (well, I did test as I went
along, but after fitting the last connection , plugging everything in and
powering up, it worked). I can type on the keyboard, the correct 
characters come up. I can use the function keys, configuration screen, etc.
 Modulo the fact that I have Danish/Norwegian ROMs in my HP120 (!)
I am now looking at modifying an HP150 keyboard to work with the HP120.
More news if I get that working!
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