Michael J Mahon built a parallel processing setup using Apple II mainboards, and a homebrew simple serial network:
At the outset, when designing NadaNet, I envisioned that it could be used to support parallel computing on Apple II machines. To add more processors and save space, I decided that I would package several Apple //e main boards together, without keyboards or peripheral slot cards.
See also the successor machine, AppleCrate II:
Very interesting.
I did something similar with Arduinos about 10 years ago, before the world went wifi crazy with the ESP8266.
There is definitely a place for lightweight protocols, and connected machines can send a series of 8-bit tokens to invoke bytecode commands.
Forth and it’s derivitives can be very concise when it comes to messaging, and if you create a very simple virtual machine that runs on any processor, you can get communication between widely different machines with minimum of data exchanged.
I’m reminded of a paragraph from a Bob Bishop biography here:
“In his consulting work, Bob became involved in a new game show on CBS, Tic Tac Dough , which started airing in the summer of 1978. This show was one of the first to make use of computers to control the game board. Nine Apple II computers were used, one for each of the nine squares of the board, and these computers were in turn controlled by an Altair 8800.”
(Why he didn’t just use a tenth Apple II for the controller is beyond me …)