[Coco] Will this build-your-own s-video board work for CoCo 3?

[gene heskett]

On Wednesday, March 09, 2011 10:58:33 AM Frank Swygert did opine:

> Yes, you can go straight to a 1084 with a straight through cable. That's
> what I used with my last CoCo3. Mine was a Commodore 1084S, but the S
> just means it had two speakers for stereo output (if you had a source).
> I don't recall the specs on the monitor, but I think it could sense the
> sync signal used. I don't recall doing anything special. I think I had
> composite and RGB connected at the same time (switch to composite for
> some games). Maybe it used the composite sync signal? Should be some
> notes on that in my book, "Tandy's Little Wonder", download the PDF
> from the list site (in folder "farna"). I don't remember now, but do
> know I made a straight through cable for it with no circuitry.
> 
This is bringing back memories Frank.  It seems the 1084, when connected 
with a straight through cable to an agnus chipped amiga, had all lines 
terminated with 75 ohms in the 1084.

The Agnus was equipted with overload shutdowns, and it could drive the 
nominally 1 volt RGB lines into the usual 75 ohm terminators just fine, but 
the sync stages were outputting std ttl signal levels.  When the circuit 
saw that load and the current it drew, the agnus shut itself down to 
protect itself.  There were in fact, a lot of amiga 2000's sold with the 
commodore 'genlock' card in them because that card had the power to handle 
an all lines terminated.  I know, I bought mine that way.  But I was using 
an 8CM515 monitor, which had a slightly sharper CRT in it, and apparently 
expected TTL level sync signals.  It worked plugged into either video port, 
but when I tried a 1084, it only gave video on the genlock cards port.  
That is when I found there was a thriving business making buffered cables 
by overloading a quad nand gate of the 7400 family, molded right into the 
plastic backshell of that db23 connector the amiga used for video.  Then we 
blew the 7400 in one of the cables at the tv station one evening when 
mother nature was using the 255 foot tall stl tower 30 feet outside the 
back door for a fireworks launcher.  

That was always a hazard there at the studios, and probably blew out at 
least a hundred audio distribution amplifiers since they had all that wire 
connected to pick up the EMP from a lightning strike looking for a ground 
when its sitting on 2000 feet of dry limestone.  I'd had some experience 
using cmos stuff at above rated voltages and currents, so that 7400 got 
replaced with a 4000 family hex invertor, with one for the inversion 
needed, and the other 5 gates all used in parallel to drive that 75 ohm 
load.  Worked great and lasted till the last amiga was retired 2 or 3 years 
after I retired.

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
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I have a very good DENTAL PLAN.  Thank you.