I've got a seperate programmer which runs off the parallel port and grabs power from the smaller of the two power connectors. The two chips are connected together via 4 pins on each. They're orientated so that one can trigger the external interrupt pin on the other, so some asynchronous communication can occur. The top chip drives the bus connector and the MAX233 RS-232 transceiver. The chips can run at 16MHz, but I'm running them off the built-in 1MHz clock.
Plans are afoot to build some clock-stuff onto the board, along with a reset button.
This chip does the RS-232 serial communications. All it really does is convert between RS-232 levels and CMOS levels. It's nicer than the smaller MAX232 since it needs no external capacitors, which makes it easier to wire up. The MAX483s drive the RS-422 serial line. This is a balanced line affair, so you can have up to 32 devices over about 1000 feet of cable.