I'm working on a 3D controller hat for a Raspberry Pi. This is a glimpse of the hardware prototype. You can see the USB and ethernet ports of the Pi on the left side.
Features:
4 motor control channels
- compatible with A4988 and DRV8825 Stepstick motor drivers
- Motor supply voltage up to 45V
3 High power (15A Max) 12v/24v discrete switch channels to drive fans, heaters, extruders, spindles etc.
6 Digital switch inputs for limit switches. Can also be used as 3.3V digital outputs
4 Thermistor input channels
4 Unassigned analogue inputs.
SPI port for expansion.
The board provides power to the Pi, and can run from 7-12V DC. It has separate supplies for the bed and motors.
The software is the hard bit (for me at least). I'm looking at a real-time raspbian kernel at the moment. The Pi will probably have to run a kernel device driver to drive the motors, but will probably be able to use the BCM2835 GPIO library for everything else.