Rasp Pi comes with I/O header what makes hackers and tinkers do lots of fun within Linux. But when you need do more specific interfacing like serial RS232 or RS485, or Servo control, it takes some preparation. To make things look more easier an expansion board called Raspy Juice was designed that holds serial RS232 and RS485 connectors and level convertors. Additionally there are four RC servo ports.
All this wouldn’t be so easy with AVR Atmega168A microcontroller on it. It interfaces to Raspberry Pi as i2C slave. After using several pins for above functions there are sill few left for general purpose. SO why not use them for analog input that are so welcome in Ras Pi. If you’re wandering what other expansion boards are already made then check out this nice list – might find something you need.