I'm working on a project to add a Raspberry Pi to my telescope setup. In the end it will do everything from plate solving to photography and also allow me to do it from the warmth of my armchair rather than standing out in the cold all night.
I'm not the first one to have thought of this and there are many ideas as to how to do it. Presented below is my take on an ST-4 compatible autoguiding interface. It plugs into the Pi's 40 pin I/O connector and sticks out of the case so as to allow the cable to connect from the guide port on your mount to the Pi. I've also added a DSLR trigger too.
I've uploaded the board to Oshpark so that you can order one (well, 3 really). The BOM is trivial and the opto chips are nothing special.
Below are the Eagle files.