[Coco] SVGA monitor
snhirsch at gmail.com
Sun Oct 25 16:09:44 EDT 2009
On Sat, 24 Oct 2009, Steven Hirsch wrote:
> On Sat, 24 Oct 2009, RJRTTY at aol.com wrote:
>>> I have questions about that. I bought one of these boards on eBay and
>>> unit I received is behaving very poorly. It resets randomly and will not
>>> display the CoCo output.
>>> Color me unimpressed.
>> It is likely the output of the coco3 is overdriving the input of
>> your converter. I have run into this problem with other
>> commercial converters. Try putting a 75 ohm resistor
>> in series in each of the RGB lines and see if that helps.
> That's possible, but I think the real issue is that this unit doesn't support
> 15.75Khz. RGB with separate V & H sync. When I connect my Amiga using the
> composite sync output it seems fine.
And that turned out to the be problem! I breadboarded a simple H+V -->
Composite TTL adapter and presto... Nice stable display from the CoCo :-)
The circuit I used was found at:
There are two variants shown there; one uses a pair of discrete
transistors to drive a true 75-ohm input and the other provides standard
+5V TTL. The latter of these worked fine for me since the converter is
Parts complement is an SN7486 quad xor-gate, two 2k2 resistors and two
22ufd/10v electrolytic caps. It's sitting on my trusty Heathkit
proto-board at the moment, but I'll end up building it on perfboard and
mounting the entire getup in a little project box.
I'm going to suggest that the manufacturer support 15Khz. RGB w/ separate
sync, but who knows if they'll care enough to listen.
More information about the Coco