I have a Roomba 530 with a serial interface and open API as specified here. Connecting the pi requires a serial connection between the pi an the roomba's serial port, and some way of powering the pi from the 18volt supply on the roomba. I have put the pi in a plastic case that is connected to the bumper. That way when it goes under a chair and catches the case, the force is transmitted to the bump sensors and the standard software figures out what to do.
Unfortunately the clear plastic bowl I could get to cut down was slightly too small and so I ended up making a 7 pin DIN plug out of a piece of brass tube, some bass wire and some epoxy putty from the local hobby shop. The process was to solder the wires onto the pins, take the battery out of the Roomba and put vaseline on the serial plug on the Roomba. Put the pins (and earth ring) in the Roomba socket, and then push putty over the top of the pins. When it dries, take the plug out (carefully) and "back fill" the plug from the pin side and push it back into its vaseline covered socket. Let it dry and clean up.
There are two electrical challenges. First the serial interface to the rpi is 3.3v and the serial interface to the Roomba is 5v. To address this I simply used a voltage divider on the Roomba TxD to rpi RxD connection (see circuit below). The rpi TxD to Roomba RxD connection is fine as 3.3v seems to be enough to drive the Roomba RxD signal.
The second challenge is that, although there 18-14v available directly from the Roomba battery via the serial plug, there is a 200mA poly fuse on that supply and the Roomba draws up to 700mA. It actually only draws 330 mA when running but round 420 on boot. The difference between what it normally draws and the specified required current is to drive things hanging off the USB ports. The solution was to use a DC to DC converter (available through Farnell in the UK) that would provide 400mA. This turns out to be enough to boot and so my 4 rechargeable batteries are redundant until I put something weighty on the USB connections. Here is the circuit I used - there are some strange choices (the inductor is surface mount; the schottiky diode is too big ...) but these decisions were based on what I had and could get. The original idea was to tune the potentiometer to deliver a trickle charge (via a current regulator) to the batteries and have the the power to the pi go via a voltage regulator. This doesn't work as I don't seem to be able to find a voltage regulator that will take 5.45v down to between 4.8 and 5.2 volts. Please mail me if you have any suggestions.
Finally the code. I have written (most) of the Roomba Open Interface
as a Java package that uses RXTX behind the scenes to communicate with
the robot. In addition I have written a user interface that provides
a "joystick" to steer the roomba, and a set of level meters that indicate
the output from the 6 "light bump sensors":
The code is in Java and uses Sockets to communicate between a computer running the user interface and the rpi, and RXTX to communicate between the rpi and the roomba. The code is available for download here as a tar ball.