Hi,

Currently I have a tethered underwater robot and the 25m tether has two pairs of conductors for serial cables (aux and can bus -- both rs 485), a pair for DC power (48V), and a pair for analog camera feed. http://i.imgur.com/GIM6JpY.png

I'm working on adding additional sensors/actuators on the robot, but I want to stay away from implementing a man-in-the-middle solution, that is, I do not want to edit or inject any of the feeds already. I would prefer to overlay a signal that isn't seen by the canbus, but can be filtered into its own signal.

I looked into power line communications, but I would like a simpler solution. I only need around 2400bps of communication. It's underwater, so a simple XBee solution wouldn't work.

Would, say, attaching the antenna of a XBee (with a floating power supply) to one of the conductors work? Or, transmitting serial on a different voltage over the can bus? Or, transmitting modulated signal on the camera line with a different carrier frequency? Or even a ultrasonic transducer+hydrophone pair? What are my options?

Thanks a lot!

EDIT: forgot to mention that the AUX cable is already taken by a non-standard weird serial communication thing.

 

I'm working on adding additional sensors/actuators on the robot, but I want to stay away from implementing a man-in-the-middle solution, that is, I do not want to edit or inject any of the feeds already. I would prefer to overlay a signal that isn't seen by the canbus, but can be filtered into its own signal.

Personally, I would look at adding CAN messages to the existing, working bus. CAN is designed to do just that. Adding messages on some sub-carrier may be possible in that you can design a circuit to run over the CAN circuit but what about the existing CAN? It is baseband and not likely to appreciate a superimposed signal. You should be able to find a group of CAN message IDs that are not used by the existing robot. If you can choose a group of IDs with a higher number than the robot uses, the robot's native messages will get through like you were never there, being of a higher priority. After that, just make sure that your messages can get through between the robot's and you are done.

Doing it like this, its not man-in-the-middle, you are just adding traffic to the bus, just like CAN is designed to do. Any decent micro-controller with a CAN peripheral (I've used the PIC18F4685 extensively for CAN) with the appropriate driver should do the job nicely.

EDIT: By 'driver' I mean the physical interface chip to match the CAN bus. Use the same P/N as the existing CAN link for no worries.

Sounds like a fun deal. Good luck.