Quote:
Originally Posted by FrankJ
Use terminating resistors. I not sure this is really needed in the short cable runs, but why tempt fate. Besides the first on the list of troubleshooting is to check the wiring.
|
Termination resistors are required for any cable length per the CAN standard. The spec is for 120 Ohm at each end, either as a single resistor or as a 2x 60 Ohm split termination with midpoint bias and/or noise filtering. Since the bus length on a typical FRC robot isn't long enough to cause a significant noise problem, the FRC documentation uses the simple single resistor option. But, if you had a 100 meter long CAN bus, then those noise considerations come into play.
CAN uses differential signaling, and those termination resistors provide the proper load (60 Ohm DC) for the drivers as well as an impedance match (120 Ohm at each end of a nominal 120 Ohm transmission line) to limit reflections on the bus. While a short cable length might not need the termination from a reflection perspective, the drivers still need a proper load regardless of the length.
The implication here is that you want a bus with the termination resistors at either end, and the shortest possible stub lengths (distance from the bus wire to the transceiver chip...which is some short distance on the Jaguar circuit board that you don't have to worry about). So, don't try to mitigate the single point of failure...if you don't have all of the jaguars in 'series' then all of the transmission line math breaks down and you'll have bus failures.
For more gory technical details here's a TI application note that goes into the bus operation and termination particulars:
http://www.ti.com/lit/an/slla270/slla270.pdf
And here are Scott McMahon's videos on how to wire a Jaguar from TI (scroll to the bottom of the page):
http://www.ti.com.cn/mcu/cn/docs/mcu...ntentId=100442
Don't ask me why it's on TI's china site, but those are definitely Scott's videos.