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

Torsten Dittel Torsten at Dittel.info
Tue Apr 5 08:08:21 EDT 2005

>    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. 

Now I see. I thought theres a (very) short piece of ribbon cable 
directly coming out of your adaptor.

> 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.

Hmmm. Hadn't thought in th AL875.

> 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.

That's odd. And I don't know enough about hardware to say that a cheap 
PAL or GAL with kind of a shift register containing the code sequence to 
  programm the devices would serve.

Bad enough that the AverLogic chips aren't saving the register settings 
without permanent current. What the heck have the designers thought when 
they made it like this? They should keep the settings (forever) and only 
on a special reset signal return to any preset values. One should revise 
the specs of the AL chips to see what video signal timing they are 
expecting and compare that to what's coming out of the CoCo.

However, if you manage to use the "RGB detection" pin, the best solution 
would be to add the init code to the BASIC ROMs (there was a discussion 
recently where there's space in the ROMs to add things. E.g. I could 
live without the picture of the Microware guys... ;-))))


More information about the Coco mailing list