You can talk to an Arduino using Firmata but several others are also available. You can talk to a Raspberry Pi, Modbus, Processing, several commercial I/O interfaces, and the RS232 port.

When you don’t want to use I2C or SPI, and MIDI and DMX are old hat, [Scott] comes along and invents a very strange networking protocol that is just daisy chaining a few Arduinos together wit… ...