[philso]

Originally Posted by wireties

This is more commonly called rise time. And you are correct, it does cause reflection issues.

I had referred to rise-time and fall-time in my original post in this thread. They both contribute to reflection issues.

Originally Posted by wireties

Thus the generic recommendation to wire it in series with a 120 ohm termination.

I referred to "actual line characteristic impedances, actual termination resistance values, impedance mismatches and actual edge-rates" because the 120 Ohms is only an ideal value for an ideal 120 Ohm transmission line. It is the non-ideal characteristics of a real world system that will cause some minor reflections even though one is using the recommended termination resistance.

Originally Posted by wireties

CANbus does not work exactly like asynchronous serial methods. CANbus receivers attempt to synch synthesized clocks with the transmitter. That is the entire purpose of the NRZ encoding and the RLL coding - to have enough transitions to phase lock a synthesized clock.

And the phase locked synchronous clock is used by the UARTs to sample the state of the bit in the middle of the bit time to avoid the ringing that can occur with each bit transition. Yes, the coding increases the number of transitions making it easier to implement the PLL. The low-level circuitry used to detect the state of each bit is not really affected by the coding. A bit is a bit.

Originally Posted by wireties

With respect using a star configuration is an unwise and strictly unnecessary technical risk. It is a good thing to teach students to follow the recommended use of communications links, sensors and other subsystems on the robot. Otherwise, a FIRST robot can quickly become unreliable. And nearly impossible to diagnose.

One of the lessons that the team members learn is management of risk. You are correct that there is risk in using a star configuration. Since there is not really anything with zero risk, the challenge is to quantify the risk and determine if it is acceptable.

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.