[Coco] Using Roy's vga adapter with other computers

Gene Heskett 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.


Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
The NRA is offering FREE Associate memberships to anyone who wants them.

This is a test of the emergency broadcast system.  Had there been an
actual emergency, then you would no longer be here.

More information about the Coco mailing list