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Originally Posted by philso
Where does this 1 Mbps figure come from?
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From looking at the CANbus driver code
Quote:
Originally Posted by philso
Most of the CAN transceiver chips, like the ones I am using at work, are rated by their manufacturers for operation up to a maximum data-rate of 1 Mbps but that does not mean the system they are installed in are running at that data-rate.
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That is because 1Mbps is the recommended maximum rate.
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Originally Posted by philso
Edge-rate is a measure (volts per microsecond) of how fast the high-to-low and low-to-high state transitions take place.
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This is more commonly called rise time. And you are correct, it does cause reflection issues.
Quote:
Originally Posted by philso
Once a CAN Bus system is assembled, the line lengths, actual line characteristic impedances, actual termination resistance values, impedance mismatches and actual edge-rates are what determine what the peak voltage and the duration of the reflections will be. The characteristics of the reflections will generally remain constant unless the system is changed in some way.
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Thus the generic recommendation to wire it in series with a 120 ohm termination.
Quote:
Originally Posted by philso
The CAN Bus receivers are connected to a circuit (usually incorporated in the microprocessor) that roughly synchronizes with the bit edges. The state of each bit is sampled in the middle of the bit period, often multiple times.
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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.
Quote:
Originally Posted by philso
Therefore, while it is best to avoid using the star configuration since it is non-ideal, it is not "certain death" to use a star configuration or to add a branch as long as one is taking some simple precautions. Because of the inherent robustness, CAN Bus systems have found many applications outside of the original intended use in cars.
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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.