23 May
2020
23 May
'20
2:49 p.m.
-----Original Message-----
From: Paul Koning <paulkoning at comcast.net>
Sent: 23 May 2020 19:29
To: rob at jarratt.me.uk; Robert Jarratt <robert.jarratt at ntlworld.com>;
General
Discussion: On-Topic and Off-Topic Posts <cctalk at
classiccmp.org>
Subject: Re: Alternative Monitor for VAXmate
> On May 23, 2020, at 9:45 AM, Rob Jarratt via cctalk
<cctalk at
classiccmp.org>
wrote:
>
> As it looks like I am not going to be able to repair the monitor board
> for my VAXmate I am wondering if I can do anything with the outputs
> from the I/O board to drive an external monitor instead.
>
>
>
> The connector to the monitor board has RGB+Intensity outputs at TTL
levels.
The horizontal
sync has a frequency of 26.6KHz, active low with the
high voltage 3.7V, Vertical sync is 60Hz. I don't believe that
corresponds to any known standard, does it?
I assume it's the same as the Pro video. That might not exactly match
standards. But the intensity output is an acceptable 480i monochrome signal. I've fed mine into a video capture device
and into a TV display,
both
work.
problematic.
I had a go at building this
http://www.dasarodesigns.com/wp-content/uploads/2014/03/pet-composite-
video- adapter.jpg feeding its output to a composite to VGA device to
see if it would convert it to VGA, but no luck.
VGA has double the horizontal frequency, so that's going to be What should work is to convert the RGB signal to
"component video" and
feed
that into a TV monitor that has such an input with
480i support. There
are
some simple circuits for doing this in design notes
from various chip
companies. One that uses a triple video amp is nice except that the
device only
comes in a microscopic size SMT package. Older
devices should work too,
though.
I have wondered about RGB, the I/O board does output an RGB signal along
with Intensity, Horizontal Sync and Vertical Sync, but I don't know if I
could feed that to a monitor that accepts RGB signals as I don't know what
the voltage levels should be. I have a VR241 which seems to have suitable
inputs but I don't know if I dare send the signals from the I/O board
straight to the VR241. I can't find a manual for it.
One complication is sync. The Pro has sync on the monochrome signal as
well
as on the green one, but if you convert the RGB to
component video, sync
needs
to be on the "Y" output and G is only a
portion of that, so it probably
won't be
good enough. The intensity signal could be used as Y,
except that in the
case of
the Pro it isn't done right -- it should be a
weighted sum of R/G/B with
most of
the weight on G, but instead DEC simply made it R+G+B
(equal weights).
I'd like to build a circuit to do this, with a sync separator, some video
amps
to
do RGB->Y/Pb/Pr and reinsert the sync. Maybe in a
few months.
paul