HP262x keyboard voltage

[Tony Duell]

> 
>  
> 
> >
> > > And using a 2392A keyboard ?
> > 
> >  I don't think I have one of those. What machine does it go with?
> 
> HP 2392A Terminal it has the same keyboard interface as the HP150.
> Keyboards look a lot like the HP150 keyboards.

Hmmm.. I don't think I have one, but _somewhere_ I have an obscrue HP 
keyboard with a 6 wire RJ11 connector. Maybe I shoukd try to dig it out.


> Yes I know and on my harddisk , very conveniant I make a lot of use of them.
> And you're right I forgot the knob, just was thinking about the looks, not
> the interface.

As you know by now, the frist requriement is that the system works. The 
correct 'looks' are very much secondary to that, particularly in a case 
like this where I am not going to modify the HP120 in any way. If by some 
great chance I get the correct keyboard I can just plug it in.

> 
> > 
> > The closest keyboard i have electrically is the HP150 one.
> 
> And not realy the right style.

Ture. 

> 
> > It is the same interface, similar circuitry, but with differnt 
> > key matrix layout. I think I can hackl that by replacing the 
> > scan counter chip in the keyboard (a 4024) with a little 
> > circuit of about half a dozen chips. 
> > 
> > The closest keyboard I have for having the right keys is the 
> > one for the
> > HP2623 terminal -- after all a similar keyboard was used on 
> > the HP125, which as you know is a very similar machine to the 
> > HP120 (to the extent that the firmware ROMs are the same, for 
> > example). Of course the interface is quite differen, but a 
> > conversion circuit shouldn't be too hard to buiold (probably 
> > cost more for case/connectors than for logic chips!).
> > 
> In a lot of cases the right HP keyboards are difficult to find even HIL
> keyboards are not always easy (reasonable priced) to find.

Now those I am not short of (both the HP46020 (individual swtiches) and 
HP46021 (capacitve membrane assembly). 

> If I was you I should go for the interface and the HP2623 keyboard, it looks
> better when using the HP120 ;-)

I'll probably do both in the end. I spent the afternoon pulling HP 
keyboards apart and buzzing out connections/ The HP2623 keyboard is much 
as I expected, and the switch matrix layout looks to be right for the 
HP120. Which means iternfacing it is just a matter of adding a 7 bit 
counter (4024 or similar) and maybe an inverter to step through the keys. 
Actually, I'll probably add some buffers too, I don't like driving cables 
from the output of flip-flop chips...

The HP150 keybaord is also as I suspected. Modifying that one should be a 
matter of removing the 4024 scan counter chip and replacing it with a PCB 
contianing a 7 bit counter an an EPROM. What makes it a little more work 
is that this keyboarsd runs at 12V, all EPROMs ruin at 5V or thereabouts. 
So I'll need to add level shifters. But that's not hard.

-tomy