This is an old post, preserved for reference.
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In the previous blog post, I started upgrading an Atari 400 by fitting a composite video modification.
Right Cart or RAM
Left Cart or RAM
I/O + OS
I/O + OS
The RAM from 8000-BFFF is shared with the cartridge slot. When the cartridge is plugged in, the RAM is disabled. It should be possible to upgrade the 400 to work like that, and with this upgrade, the two machines should be functionally identical. The main address decoding on the Atari 400 uses a 74LS42 chip, a 4-10 decoder, a slightly unusual choice over the ususal 74LS138 3-8 decoder, especially as there is already a 138 on the BOM, so it would surely have been cheaper to use two of those instead?
PET ROM/RAM boards to add the extra RAM, but the position of the CPU board precludes the use of a plug in daughter board as it is too close to the edge of the metalwork. I then briefly considered a replacement CPU card which included RAM, but there's a bit too much stuff going on. Also the schematic I had for the CPU card doesn't match the boards I have (they appear to include 245 buffers on the address bus which are not necessary with the 6502C version of the CPU that has the additional halt pin).
I've been looking at alternative options, and I've designed a 48K RAM cartridge. This will not work on an unmodified Atari 400, but all it will take is four wires to be soldered between the cartridge port and the RAM slot, but it should be acceptable.
You can clearly see the damage at the start of side 1 and the end of side 2. So I spliced the start of side 2 with the end of side 1 and created a new wav file.