[wireties]

Originally Posted by philso

For an example, consider a branch on a CAN Bus that is 10 ft long to reach a Talon SRX that is mounted on some arm on the robot using the same wire as the rest of the system. With a 1 Mbps data-rate, the bit time is 1000 nsec. With a rule of thumb value for the propagation time for a pulse down the transmission line of 2 nsec/ft, a 10 ft long branch would cause a reflection that returns in 40 nsec. after the bit transition that caused it. Assuming that the proper termination resistances at the RoboRio and the PDP are in place, it is reasonable to expect that the amplitude of the reflections would have diminished to become insignificant by the middle of the bit time, 500 nsec. after the bit transition.

Based on the calculation shown above, I would determine that the risk of using the 10 ft long branch is minimal. Since each person/team is willing to accept different levels of risk, I can understand if you choose not to use such a branch.

A better choice is to put the CANbus termination at the top of the arm. If you had two such arms live with the increased wire length. CANbus can be 40m long, plenty long enough to wire anything on a FIRST robot properly. Lesson #1 in risk management is to avoid risk whenever possible.