Slightly OT: Old CRT displays vs. new LCD

Tony Duell ard at p850ug1.demon.co.uk
Mon Dec 3 18:03:42 CST 2007


> This reminds me -- I'm getting a CoCo 3 very shortly in a trade. From
> reading the specs, it looks like the RGB video output should work with a
> Commodore 1902 or 1084. Anyone used it with that?

I have no idea. What I can describe are the (NTSC-version) CoCo3 video 
outputs.There are 3 of them

The first is composite NTSC baseband video on an RCA phono connector. It 
should feend any NTSC (RS-170 rate) monitor

The sccond is an RF output on Channel 3 or Channel 4 to feed into a TV 
set aerial (oops 'Antenna') connection. Again an RCA phono socket

The last is the one you'r etalking about, I think. It's a 10 pin header 
(2*5 pins) on the bottom of the machine/ A standard IDC header socket 
will fit, it's the same sort of connector as wsa used for the 'second' 
RS232 port on a lot of ISA cards.

The signals are line-level audio (let's forget thet, it's also on a 
separate RCA phono socket on the back of the mahcine), TTL level sync, 
and analogue (1V level) RGB video. No sync-on-green or anything like 
that. I beleive the video signals are designed to drive 75 ohm loads.

What I discoverted ('For I have a kludge and a good kludge too' ;-)) is 
that if you _don't_ terminate the RGB signals, they will drive TTL inputs 
(!). OK, you only get 8 colours (on/off for each of R,G,B), and you have 
to fiddle with the colour look-up table to get the signals either hard on 
or hard off, but it does work. Certainly things like white 80-column text 
on a blue background are very useable. I drove a CGA monitor from my CoCo 
3 that way, with a little buffer box to boffer the video signals and sort 
out the sync.

-tony



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