[Coco] Using Roy's vga adapter with other computers
gene.heskett at verizon.net
Sun Oct 25 12:43:32 EDT 2009
On Sunday 25 October 2009, Steven Hirsch wrote:
>On Sun, 25 Oct 2009, Gene Heskett wrote:
>>> Any hints on where to get parts for creating the cable needed?
>> Keep in mind when srcing the parts, that the amiga's video connector is
>> _not_ a db25, but a db23.
>The DB23F required for the computer end of the cable is rare, but there
>are a few folks on the internet selling them.
>> Also keep in mind that the agnus chip cannot drive all 5
>> outputs and will shut down to protect itself if the H&V sync lines aren't
>> buffered with external logic. All of the cables made back in its heyday
>> actually had a cmos logic chip buried in the backshell plastic casting to
>> not only buffer the synch signals but also combined them into whatever
>> mode and polarity that monitor needed.
>But, the Amiga connector supplies both H&V sync and composite on three
>separate lines. Why would you need further munging?
Because as I said, the agnus will shut itself down if it finds a std 75 ohm
load on the synch signal pins. It doesn't over heat, it just shuts down.
>> It being cd4000 family cmos stuff, wasn't all
>> that happy driving a 75 ohm load, so the failure rate was a problem, I've
>> had to cut it off and build a new one twice now. And I used the 74HC
>> nand gate for that, much more robust.
>Is that what's inside those silver HD15F --> DB23F adapters that were
>provided with newer Amigas?
I believe there was a similar lashup in those, although I don't know if the
alice chip had similar shutdowns built in. Mine is an A2k, and has a
picasso-ii card which drives vga/svga monitors directly.
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