Find a layer 2->7 managed switch and traffic shape on the robot
At the baseband non-packetized:
Differential RS422/485 would allow a bus where a device on the bus could disconnect or 'vampire tap' without killing the whole bus.
http://en.wikipedia.org/wiki/Vampire_tap
http://en.wikipedia.org/wiki/RS-422
http://en.wikipedia.org/wiki/EIA-485
However if you intend to run star configuration instead of a bus you may as well just use Ethernet and a switch.
The effort involved is really pretty similar and the conversion of the signals to differential built into most of the Ethernet devices.
Generally I think working around the broken wire issue is not entirely possible because sooner or later the FIRST control system has a few single points of failure.