Martin Eberhard got me the schematic for the GR 721, which is very similar
-- I traced the pinout on the DB25 and it's correct for the 771. In case
anyone else needs it:

The 771 needs +5 and -12 for its keyboard controller and the mix of 7400
series logic on it. There are apparently versions that take unregulated
power, and even some with a built-in AC power supply. It's got internal
jumpers for selecting strobe (+/- pulse, +/- level). The lower 7 bits of
the ASCII data is gated onto the bus by the strobe, but the high bit
(parity) is always being fed out.

Anyway, I have it hooked up to a bench supply for power, and jumpered to
the Polymorphic VTI's keyboard port. It reads fine, there's still something
going on with the CPU board though.


On Mon, Mar 12, 2018 at 1:13 PM, dwight via cctalk <>

> What kind of parts does it have? If it is all ttl, you only need to
> connect 5 volts and ground. There will be a strobe, 7 or 8 bits of data and
> possible wires for repeat ( usually these go directly to the keyboards
> repeat switch, without logic.
> Older keyboards required a negative rail. This was usually for a ROM or
> With ttl parts, finding power and ground is easy. A little checking with a
> scope and ascii table will quickly find the rest.
> Many keyboards had stable data for both positive and negative edges of the
> strobe so that may not be an issue.
> Dwight
> ________________________________
> From: cctalk <> on behalf of Chuck Guzis via
> cctalk <>
> Sent: Monday, March 12, 2018 9:27:33 AM
> To: systems_glitch via cctalk
> Subject: Re: GRI 771 Keyboard Info
> On 03/12/2018 07:44 AM, systems_glitch via cctalk wrote:
> > All,
> >
> > I'm looking for info on a George Risk 771 parallel ASCII keyboard. I've
> got
> > one but no documentation or existing cabling. I'd like to interface it to
> > my Poly-88.
> >
> > Thanks,
> > Jonathan
> >
> Wasn't there something about that keyboard in the Apple I documentation?
>  I can't recall exactly, but it sounds familiar.
> --Chuck

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