ProCoCo cart: Propeller-based Tandy/Radio Shack Color Computer 2 cartridge

If you’re into retro computing, this project may be for you. Plainsteve developed this custom cartridge for the Tandy/Radio Shack Color Computer 2 (CoCo 2) last Fall and released the details recently on his blog. The project is based on the Parallax Propeller and unlike other CoCo cartridges which simply play a program from ROM, this cart breaks out access to the CoCo’s memory, allowing control of the machine to be taken over by the Propeller.
He writes:

With the ability to read and write the CoCo’s memory system, complete control of the system is gained. It was fairly easy to test memory transactions, starting with writing a byte to the screen (which at power-up defaults to memory location $0000, if I recall correctly) and seeing if it changed. Once writing seemed to work, testing reading was simply a matter of checking if a written value was the same when read. 

The schematic is provided as well as a link to the ProCoCoTest.spin source code.