ClausB

According to the Atari 5200 Service Manual, there were two different versions of the four port model, as far as the circuit board goes.

Neither the expansion port nor the cartridge port, by itself, is really RAM capable. The expansion port lacks enough address lines - you could bank switch but only with very small banks. The cart port lacks write and timing signals, though you could kludge a RAM as was done for the 2600.

Together, the two ports would support a decent RAM upgrade. Put the RAM in a cartridge and run a small cable back to the expansion port. With 16K bank switching, a la 130XE, you could add as much RAM as you need, without internal mods, as Rybags said.

Rybags, on Thu May 10, 2012 8:12 AM, said:

I've thought about it some too. The 800 uses PIA signal CB2 to generate -COMMAND and it uses SKCTL mode 2 to transmit data and 1 to receive. The 5200 has no PIA, of course, but if we use SKCTL mode 6 to transmit, then data gets clocked out at one rate and we can output a signal at a different rate on CLOCKIN. If we wire CLOCKIN to the drive's -COMMAND and program it to generate the one long pulse, then we get all we need without adding a latch nor any active circuitry. Haven't tried it but I think it should work.

As you said we need a ROM cart for the SIO program and a mini-DOS. It could be a read-only DOS just for loading 16K games into RAM, or it could have some write capability for saving game states or scores.

And like you said the cart could also contain RAM. We would have to wire two signals (R/W and Ph1) from the expansion port to the cart to support the RAM. 16K or 24K of SRAM would give lots of room for bigger games.

Maybe we don't need it today, but it could have been something back then.

Congrats! When you open your 400s don't forget to reseat the chips. That is, gently pry each chip part way out of its socket from both ends and push it back in. Use a small thin blade screwdriver. That has fixed my 400s several times.

XEGS console only, filthy, broken, but cheap. No sockets - will be hard to troubleshoot.
Most of its chips have 1987 datecode except ANTIC and GTIA which are from 1983. They must have had piles of old stock to use up.