Since the Xilinx XC95144XL chips are not that 5V tolerant, and some cartridges tie 5V line directly to RD4 and RD5 inputs, some provisions have to be taken. If i knew about this (they failed to mention it in the datasheet) that fix would have been incorporated on the Ultimate1MB board, but I didn"t.This is a necessary step, and failing to accomplish it will lead to failure of the Ultimate 1MB board.
Basically you need to cut 2 traces, and patch them back together using resistors from accessory kit.
1200XL is quite diffrent from any other 8bit Atari, it has split OS rom (2 chips 8kbytes each) and no BASIC. There are some jumpers to reconfigure it to use with signle 16kbytes OS ROM chip though, so here we go -1200XL
Remove W9, W13, W8, W12 and W7, insert W11 if it"s not already there, also - and this is very important - remove W6 - this one is near U13, just right to R41 (see the picture)
Cut RD5 trace leading from pin 14 of cartridge connector, to pin 8 of MMU. Make sure you"re cutting the correct trace, and you"re doing this AFTER the VIA (away from resistor connection) on the board.
Now use provided resistor to reconnect RD4 and RD5 signals as shown. After this you may want to power up your Atari again. It should work as usual including all cartridges.
The very last thing you need to do is, solder four wires from the DuPont connector you prepared above - these are PHI2, RW, RESET, and HALT signals that are not present on MMU or OS ROM chips and have to be taken from somewhere else to get the board operational.
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