Re: RGB monitor technical questions

[dempson@actrix.gen.nz (David Empson)]

<aclark@deathtospam.envirolink.org> wrote:

> I have been trying to use a Magnavox RGB Display 80 on both an Apple //e
> (with AppleColor RGB card) and an Apple ///.  I have been unable to get
> the monitor to work with either machine.  On the //e, normal text
> displays as yellow text on a red background.  The card is set for black
> background and white text.  Graphics don't work at all.

This could be due to an error in the cable, with the wrong signals
connected.

There are four video signal lines from the Apple ///.  They can be
decoded to select 16 colours, but you would be getting the wrong colours
if the monitor is trying to use other pins.

> I noticed something on the back of this monitor that makes me suspect
> that it might be incapable of handling RGB output from the //e or ///.
> It says that the RGB input to the monitor is for TTL (Transistor to
> Transistor Logic) RGB signal.  But my Apple /// Owner's Guide mentions
> that the Color Video output port in the back outputs seven signal types
> simultaneously, only one of which is TTL RGB signal.

Where exactly does the manual say this?  I have an Apple /// Owner's
Guide here (dated 1981), and I can't see anything resembling this on
page 104, which has an overview of the connector, or in Appendix C,
which provides full details.

The only thought that occurs to me is that you might be counting all the
pins that mention video output, of which there are seven.  When
connecting to an RGB monitor, the only signals used (other than ground)
are the four XRGB outputs and possibly SYNCH.  BWVID and NTSC are not
used.

A TTL logic signal by definition only has two levels.  If there was only
one TTL RGB signal then you could only get two colours out of the
machine (e.g. black and white).  Four pins (as there actually are) gives
you sixteen combinations.

The diagram on page 134 shows a rather complex resistor network that can
be used to derive analog RGB signals from the XRGB outputs.  This
network (or equivalent) should be already present inside the monitor.

In theory, all you need to do is identify which four pins the Magnavox
expects to find the TTL RGB signals on, and connect these to the
corresponding XRGB pins on the ///.  (You will also need the GND pin,
and might need SYNCH.)  If the four XRGB pins are connected in the wrong
order, the displayed colours will be wrong (but black and white will
still be correct, because they are colours 0 and 15).

> On the //e side, I know very little.  I'm not sure what type of RGB
> signal the AppleColor RGB card produces.

If it is the card I'm thinking of, then it should produce digital (TTL)
RGB with the same pinout as the Apple ///.

On the other hand, the card might be producing analog RGB, which would
not work with this monitor.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand