[MikeE]

I was experimenting with the CAN bus on our spare electronics board earlier and discovered some characteristics caused by termination problems which might be helpful.
I'm not claiming this is the cause of many, or indeed any, CAN issues for other teams but it is one potential failure mode.

Our practice board uses the documented terminator of an RJ-12 with a 100ohm resistor crimped between pins 3 & 4. (We also added a short length of telephone wire crimped into pins 1 & 6 as a handle for insertion / extraction which I don't believe has any bearing on the issues.)
DMM testing showed a ~100ohm resistance as expected. However at some stage the resistor leads had bent towards each other to the point where they were almost touching, creating a potential short on the CANH & CANL lines. Mechanical shock could potentially cause them to touch for an instant.

I discovered that when the lines were shorted even briefly the CAN bus failed completely. Interestingly, removing the short did not restore CAN, and neither did removing the short and rebooting the cRIO. However removing the short and power-cycling the robot did restore CAN. I predict that just power-cycling the Jaguars would have the same effect, but I didn't test this (although our code would still need to be rerun to initialize CAN properly).

We are using serial-CAN so we could still control the bridging BlackJag using RS232 while the CAN bus was out. I conclude that only seeing the bridging Jaguar is a useful diagnostic indicator for a potential terminator problem - sorry 2CAN users.