[Coco] Re: [Color Computer] Re: Adapter from Coo3 to SVGA

[RJRTTY at aol.com]

In a message dated 4/5/05 3:19:19 AM Eastern Daylight Time, 
Torsten at Dittel.info writes:

<< > The only
 > problem I can see is that I wont be able to use the "standard"
 > monitor cable people may already have (and you can get from
 > cloud nine).   Not enough pins.  Cable has 6 pins and we
 > need 7 unless someone knows how I could multiplex the PIA
 > signal with one of the RGB signals.
 
> I don't understand that. Isn't your adapter connecting to the CoCo's RGB
> port and a standard VGA monitor is connecting to a d-sub15 type
> connector on your adaptor? >>

   The cable connecting the coco3 to the converter is terminated
with a six pin DIN plug on the converter end of cable.   This plug
is inserted into a six pin DIN socket which is the input of the converter.
All the RGB signals available on the bottom of the coco3 are
connected via 10 position IDC socket to the ribbon cable
but the line for the #10 pin which is the PIA input is not connected
to the DIN plug on the other end. 

        The SVGA monitor does plug into a standard VGA socket
at the output of the converter but it has nothing to do with the 
DIN socket at the input of the converter.
 

>      The display is there after power up when you hit the reset button
>  on the coverter but it does not give a readable display, just a green 
"blob".
> Programming is necessary  because the default values
> for the registers in the video chips aren't compatible with
> the coco3's video output.

>Strange. What kind of video signal is it expecting? What's not standard
>of the CoCo's video signal? Which parameters of the AL250 are you
>changing to make it readable? Could it be that beast is starting up in
>component video mode and you have to change to RGB mode? I thought this
>could be controlled by the pins of the AL250 and not just by software...

>Torsten

     Well, as far as the AL250 is concerned, the default
horizontal
position of the image is wrong leaving it off center too far
to the right.
    The AL875 has a register containing the
number used to divide the reference clock in order to
generate an output signal which is line locked using
a PLL to the incoming horizontal sync from the coco3.
The default value of this register is wrong for the coco3
and leaves a garbled display.

Believe me, if there was a way to avoid the need to
program the video chips I would have used it.     When
I started this project, I had hoped the default values
would suffice but that wasn't the case.  I don't know
what video standard the default values conform to
(if any) but they don't fit the requirements of the coco3
video signals.

Roy