12 Mar
2018
12 Mar
'18
12:01 p.m.
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:
http://deramp.com/downloads/mfe_archive/030-Keyboards/George%20Risk/GR%2072…
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.
Thanks,
Jonathan
On Mon, Mar 12, 2018 at 1:13 PM, dwight via cctalk <cctalk at classiccmp.org>
wrote:
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
EPROM.
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 <cctalk-bounces at classiccmp.org> on behalf of Chuck Guzis via
cctalk <cctalk at classiccmp.org>
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