DemUinoTVout is an arduino-based computer that uses two atmega328s, a PS/2 keyboard and a composite video signal to drive a monitor/TV with the TVout library/interface.
The interpreter and PS/2 library use the primary controller that communicates using text via the uart port with the secondary that handles the video interface. Programs use the internal EEPROMs of both controllers. Arithmetic and mathematical expressions use RPN with a stack of 4 and a number of tokens of 10..
Step 1: Schematic
The schematic shows the configuration that uses a standard RS232 driver for programming. This is not mandatory and indeed you may use a USB-to-Serial ftdi module to achieve this goal. However, the switching circuits with the two RX/TX signals and the isolation resistors between the uarts is required. Normal communication uses the center position of the switch. In order to program either controller throw the switch accordingly.