Ok after reading the manuals JonteP scanned (see http://forums.bannister.org/ubbthreads.php?ubb=showflat&Number=70650#Post70650
) and checking disassembly:
* The "Specifications and Installation Guide" chapter of the owner's manual doesn't have any extremely useful circuit-level schematics, but does hint at what processors run slave to which others, and what should connect to where.
* I'm fairly certain that 0xE6 read that happens on the maincpu is waiting for one of the two other cpus (the fdc?) to finish its self test and set a status flag before the main cpu will run. Despite what the manual says I have not found a maincpu or subcpu rom checksum test yet, but they may well exist. The fdc cpu clearly has one and it passes.
* I'm also not sure where the EAROM 'eeprom/nvram' chip maps to, nor am I sure it is dumped yet. I hope it still works, as those chips have a bad failure record on atari arcade boards.
* The main cpu is responsible for the serial interface ics, PROBABLY responsible for the interrupt controller, and the other two cpus run slave to it.
* The sub cpu should more properly be called the display cpu, and more or less implements a complete terminal of sorts within the system, and is also responsible for talking to the keyboard.
* The keyboard is rather complex, and almost certainly has a processor or mcu inside which is not dumped.