[Coco] RGB video on CoCo3

[jdaggett at gate.net]

On 14 Feb 2009 at 16:34, Phill Harvey-Smith wrote:

> Steven Hirsch wrote:
> > On Wed, 11 Feb 2009, Phill Harvey-Smith wrote:
> > 
> [snip]
> > 
> > The main problem you're facing is that RGB SCART requires composite
> > sync, while NTSC CoCo 3 systems (and maybe PAL, but I've never seen
> > one) have separate H & V sync.  I ran into this trying to interface
> > my CoCo to a scan converter that uses SCART.  If you're handy,
> > there's a simple circuit to recombine H & V to composite.  I plan to
> > knock this together Real Soon Now and play with it further.
> 
> Ok some progress, I made up some leads to go from CoCo to veroboard
> and veroboard to SCART so I can do some patching. So far I have :-
> 
> RGB on CoCo direct connected to RGB on SCART (plus grounds connected
> as needed).
> 
> H & V Sync on the CoCo XOR'd together with an LS86, and the the output
> inverted with an LS04 and passed to CSYNC on the SCART.
> 
> This produces a stable picture that is pretty crisp *BUT* in shades of
> almost white, Ii.e. the colour is all wrong :( I tried putting some
> resistors in line incase the voltage out of the CoCo was the wrong
> level, seemed higher than 0.7V when measured with my scope, looked
> more in the 2V range.
> 
> Any thoughts anyone ?
> 


The voltage from the Coco VGA is in the range of 0.8 VDC to 2.0 VDC. SO 
if you are seeing voltage maximums around 2.0 VDC then that would be 
correct. Remember the Coco3 was designed to drive the CM8 monitor or 
vice versa.

To drive a different monitor that needs lower levels then I suggest using a 
resister divider network and then buffer that with an op amp to drive the 75 
Ohm impedance of the monitor input.  do not try and use a resistor divder 
then feed that to the monitor. That will screw up the impedance match. 

james