I am working on a robotics project using PICAXE 18X microcontrollers. The robot that I'm designing will work in two parts -- a base unit, which on it's own will simply have obstacle avoidance programming, plus the code to control the drive train, and various "backpacks", which will contain other code to "specialize" the bot in various ways. Each backpack will contain it's own microcontroller. What I want to do is have the backpacks essentially jumper two pins on the base micro, to force the base unit into I2C slave mode, but still allow the 'bot to function with no backpack installed. Ideally, the drive train code would remain on the base unit itself, and the backpack micro would simply call subs from the base unit. Unfortunately, as I am still a complete newb with regards to microcontroller commands, I have no idea how to do this -- I can't find any documentation about how to use one micro to control another in this way. Any help, suggested reading, or code snippets would be greatly appreciated.