Making the Atari 800 Compatible with The!Cart, MyIDE II and more
Both the Atari 400 and the Atari 800 were conceived at the same time are and basically identical. One major difference between the Atari 800 and the Atair 400 is the presence of the right cartridge slot. This slot was not a big commercial success and is not there in the Atari 400. It is also not there in any of the successor models like the Atari 1200 XL, Atari 800 XL, Atari 130 XE.
On pin "S" of the cartridge slots of an Atari 800 there is not the PHI2 signal attached as on all newer machines. Instead there is RAS signal connected. The falling edge of PHI2 and RAS is nearly the same, but RAS is much shorter low than PHI2. The logical, since there be some time for the CAS signal, too. This is the comparison of the signals on the scope:
This can lead to problems with modern circuits. Most modern circuits in flash cartridges or flashable cartridges sample the PHI2 signal for their logic. Such cartridges will run into problems on the Atari 800. They will either not work at all or run into errors during write cycles. This includes for example The!Cart and MyIDE II.
The most viable solution is a small modification that makes the Atari 800 compatible with the Atari 400 and the XL and XE series. This modification connects PHI2 instead of RAW to pin "S". No problems have been reported running any kind of Atari 800 cartridge with the modification installed. Even original cartridges that write the cartridge like "Telelink II" still work fine. If you want to do this patch by yourself, here is the easiest way.
Then connect pin "A" and pin "S" on the "right" cartridge slot together as shown with the yellow wire. Then cut the RAS trace at the location marked on the right.
That's all. This mod is independent of which TV system you have, it works also on NTSC as on PAL or SECAM.
If you have further questions, you can post in the related AtariAge thread.