[Coco] rgb2vga

Zippster zippster278 at gmail.com
Tue Sep 20 11:19:54 EDT 2016

Hi Mathieu,

R, G, and B are taken in on pins 1,2,3 as usual, composite sync on pin 13.

Here’s the routing for the board I did (Luis’s original board has the same circuit).
<https://drive.google.com/open?id=0B1sgl615FlGnWmhISkRVZkRjU0k <https://drive.google.com/open?id=0B1sgl615FlGnWmhISkRVZkRjU0k>>

As you can see, there’s no separate hsync and vsync on the connector, so for the
C128 RGB port, you would probably want to run some patches between the RGBI and
analog RGB connectors.  You’ll also need to patch the intensity line (with a voltage divider)
to the FPGA IO header as detailed on Luis’s site.

An easier approach is probably to have one end of your cable a 10 pin header connection
and just use the analog port.

A note of caution…  as you probably already know, the C128 will be putting out TTL level
signals, so you’ll want a series resistor on the RGB lines (to act as dividers with the already
present terminating resistors on the board) in order to drop the voltage to levels that won’t
harm the FPGA IO.  Also be sure you’re running with the ground fix on the hsync divider, for the
same reason.

You may have to adjust the FPGA programming to sync the pixel clock with the one in the C128
more closely for the intensity signal to be applied to the video conversion in a stable manner.

This issue could be avoided by using some logic and a simple resistor DAC to create pseudo-analog
signals from the TTL inputs before sampling.

I hope that helps, and good luck.  :)

- Ed

> On Sep 20, 2016, at 9:17 AM, Mathieu Chouinard <chouimat at gmail.com> wrote:
> On Tue, Sep 20, 2016 at 9:48 AM, Robert Gault <robert.gault at att.net> wrote:
>> Mathieu Chouinard wrote:
>>> what is the pinout of the rgbin connector?
>>> Mathieu
>> 9  7  5  3  1
>> .  .  .  .  .
>> .  .  .  .  .
>> 10 8  6  4  2
>> RGB pins on Coco3
> sorry I was talking about the DB15 RGBIN connector.
> I'm trying to build a cable for a commodore 128
> -- 
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