Could the Espruino be made into an I2C slave device?

Interesting idea... I think things like SPI/I2C slaves are a bit tricky because you basically need an instant reply when data is requested. Something could be added in native code I guess, or if we could think of a nice way to expose it as an API then slave support could definitely be done.

Probably the easiest way to connect them together is to use serial though - I can think of two pretty easy ways:

• chain RX to TX along all the different boards
  
• have a single wire, pulled to 3.3v with a resistor - then connect ALL RX and TX lines together and set RX to 'input' and TX to 'output_opendrain' - then every board can send messages to every other board, using a single wire.