[Kevin Sevcik]

So, first and foremost, there's still some reason to use the CAN-bus. You can still offload some functionality to the Jags, such as limit switches, potentiometers, encoders, and current sensing. Yes, your feedback on encoders and other sensors will be delayed, relative to what you'd get on the cRIO, but it is an option.

Second, even without the sensors and closed loop control, CAN-bus is still makes some sense. Provided you're not using the serial adapter, but the ethernet-CAN adapter. You can send commands rather faster over the CAN-bus than you can over PWM, and they'll be more stable with better resolution. So I wouldn't let this ruling dissuade you from using CAN. If you're aiming for tight, fast response control loops, then it's still the way to go.

Third, I'm also mighty confused by this restriction from the GDC. Especially if the 2011 FRC firmware for the Jags still supports closed loop control modes. (I haven't gotten a look at it yet.) We use independent servo controllers all the time in industry, and they're perfectly safe. They're all designed with various disable mechanisms to safely shutdown the servo in the event that communications with the host controller are lost. Near as I can tell from the reference code, the Jaguar is set up the same way. So you'd still have the exact same level of safety in the system as you do with the PWM inputs. If offloading control to the Jag was unsafe somehow, then CAN-bus commands wouldn't be any safer. If the Jag failed to disable closed loop on loss of signal, it's still going to fail to disable PWM control on loss of signal.

The only theory I've got is that they're really wanting to stick with good, general application rules, with as few exceptions as possible. We can all admit that the most problematic parts of most rules are the one or two exceptions specifically in the rules. You do have to admit that the blanket ban on external command signals is some fair bit simpler than a ban with an exception for Jags, or Jags with XX firmware, or external control loops that aren't vetted by FRC, or external control loops that don't meet the following safety protocols, etc. etc. etc.