Alternative Monitor for VAXmate

[Rob Jarratt]

> -----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
video
> signal.  I've fed mine into a video capture device and into a TV display,
both
> work.
> 
> > 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
problematic.
> 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