[Coco] Is MM/1 to SCART practical?

[Barry Nelson]

 Ok, if it works with a Tandy CM8 monitor, then that means that the VSync timing, the HSync timing, and the video signal voltage are all what a SCART device expects to see, so there is hope. However, it also means that if you do not have a composite video out, you may have no composite sync signal available since the CM8 uses a separate vertical and horizontal sync. To make things more complicated, the sync to the CM8 is a positive sync, and SCART needs a .7V negative sync. You will need to invert both of the sync signals and combine them together and then connect that to pin 20 on the SCART connector. This circuit MIGHT work if you add 330 ohm resistor to the composite sync output of the circuit to bring the voltage to the proper level.

http://www.twobits.com/cga.gif

Issues: The resulting composite sync signal will lack the "front porch" and "back porch" plateaus that a composite video signal has. This may result in dark or washed out video.
Solution:  The AD724 IC chip should be able to generate a full composite video output for you from the signals you have, as well as an s-video signal, which may give you what you need without further conversion as s-svideo should provide good resolution at 640x225 480i.

Here is the data sheet on the AD724, which contains a sample circuit to convert VGA to s-video which may work for you.
http://www.farnell.com/datasheets/83869.pdf

You will not need the lower part of the example using an AD8013 chip which is to drive a VGA monitor and can simply tie the pin 5 on the AD724 to +5V. You will need a 3.579Mhz crystal in addition to the AD724 to attempt this solution.

Obviously I have not tested either of these solutions on an MM/1.

On Aug 23, 2015, at 1:18 AM, coco-request at maltedmedia.com wrote:

> Date: Sat, 22 Aug 2015 21:56:00 -0500
> From: David Ladd <davidwladd at gmail.com>
> To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
> Subject: Re: [Coco] Is MM/1 to SCART practical?
> Message-ID:
> 	<CAHW2jkBQufKJKJXHo=AkeoB27pzFT5sQzZ4Q3DbUUygRwWqqdA at mail.gmail.com>
> Content-Type: text/plain; charset=UTF-8
> 
> Well in this case I don't have to much data from the documentation I have
> about the video signal stuff.
> 
> I know I can use a CM-8 monitor on my MM/1 without any problems.  I also
> use to have a Magnavox monitor I used on the MM/1 and CoCo3.  Sadly the
> Magnavox died first and then my CM-8 died not long after.
> 
> So that is one of the reasons I got the RGB2VGA hoping I could use it on
> both.  The SYNC seems to work on the RGB2VGA, but the colors are so dark on
> the VGA monitor with the MM/1 I can't read the output.
> 
> The other thing is I don't have the tools to test all this stuff.
> 
> That is why I was hoping someone out there with a MM/1 might know or have
> more experiance with this.