[Dan] has been hard at work developing CYNCART to get his Commodore 64 and original NES to play together. We’ve seen [Dan's] handiwork before, and it’s pretty clear that he is serious about his chip tunes.
This project starts with something called a Cynthcart. The Cynthcart is a Commodore 64 cartridge that allows you to control the computer’s SID chip directly. In effect, it turns your Commodore 64 into a synthesizer. [Dan] realized that the Commodore’s user port sends out simple eight bit values, which happens to match perfectly with the NES’ controller ports. In theory, he should be able to get these two systems communicating with each other.
[Dan] first modified the Cynthcart to send data out of the user port on the Commodore. This data gets sent directly to the NES’ 4021 shift register chip in the second player controller port. The NES runs a program to turn this data into sound on the NES’ audio chip. The first player controller can then be used to modify some other sound settings on the NES. Musical notes are played on the Commodore’s keyboard. This setup can also be used to play music on both systems at the same time. Be sure to watch the video of the system in action below.