Jaguars Do Not Connect to BDC-COMM

[Bryscus]

Originally Posted by jwakeman

It sounds like you have a lot of experience with CAN. In your opinion is it reliable enough for competition? Were the issues only after the initial wiring and test? Or are these the type of things that will plague you on the field?

Mr. Lim suggests some good things to check. Follow the list and you should find your problem.

We've used CAN for a couple of years now and despite some weird problems that we run into, it is pretty dependable. More often, the Jags are the limitation, not the CAN bus itself.

Some issues to consider (CAN only potential problems):
- When using a Jaguar in a closed loop mode (speed, position, current), if the Jaguar shuts down for whatever reason (overheating, overloading, tripped circuit breaker) it will reboot in open loop voltage control mode. It will then ignore any closed loop command signals you send it because it is looking for voltage commands.

- Jags in general seem to be more susceptible to FOD (foreign object damage) such as metal shavings and such. You must be VERY careful.

- If cables are made poorly, you can get intermittent connections - however this is not limited to CAN.

- One really NASTY one to consider is that you can't really pull the breaker on a CAN jag, you must disconnect the motor instead. The reason for this is that the CAN bus is a large network. Each Jaguar has its own address (0-63). ALL the devices on the CAN network see the same data, but each device ignores any commands that aren't meant for it. The code in the cRIO sends a packet(command) and expects to hear back from that Jaguar. If you pull the breaker on the Jag, the cRIO code will keep trying to contact the Jag and will send a bunch of errors to the NetConsole. Get enough of these errors and the whole robot will do some funny things (because the FRC program can't execute fast enough). Also, the same issue applies with losing power to closed loop controlled Jags above.

That being said, if you don't have any reason to use CAN other than it reduces wiring, I wouldn't waste the effort. However, if you need to use closed loop modes, synchronous updates (updating all Jags simultaneously) or you just can't get over how pretty the black Jags are then the effort may be worth it.

- Bryce

P.S. I don't mean to scare you away. I just want to make you aware.