Who understands the Atari 7800 A12 timing kludge?

[supercat]

Some Atari 7800s have a timing kludge which was supposed to fix Dark Chambers but broke quite a few other games. I've seen the schematics of the kludge, though none of them seem quite right; it seems there should be both a resistor and a cap in there. Does anyone understand in detail how Atari's kludge works, or what waveforms it generates on A12? My 7800 doesn't have the kludge, so I can't test such things.

[BigO]

Some Atari 7800s have a timing kludge which was supposed to fix Dark Chambers but broke quite a few other games. I've seen the schematics of the kludge, though none of them seem quite right; it seems there should be both a resistor and a cap in there. Does anyone understand in detail how Atari's kludge works, or what waveforms it generates on A12? My 7800 doesn't have the kludge, so I can't test such things.

How does one know if their 7800 has this kludge?

[+Mitch]

Check out this thread from Google Groups.

 

Mitch

[supercat]

Check out this thread from Google Groups.

 

Well, the schematic in that one is a bit of a mess, but it says that A12 starts low for about 200ns. That still leaves open some questions, but may be enough information to make things work on some potential new banking schemes.

 

Of course, I suppose if I had a 7800 with the kludge in it I might be better able to test my new schemes, but knowing what to expect should help in any case.

[+Mitch]

Try here for the schematic. (scroll almost to the bottom)

 

Mitch

[supercat]

Try here for the schematic. (scroll almost to the bottom)

 

I've seen the schematic, but I don't quite understand how the cap is working without any pullup or pulldown on the other side of it. Is it relying upon TTL logic gate inputs to have a significant inherent current sink?