Quote:
Originally Posted by wireties
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.
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Yes, it would be better to put the termination at the end of the longest branch. I am not sure if the termination resistors in the RoboRio and PDP can be disconnected easily so this might not be possible.
Quote:
Originally Posted by techhelpbb
As I am a CSA: I usually make my recommendations and explain that the default recommended configuration includes ample consideration of system traits, that a star configuration adds risk.
If a team proceeds, has issues, and refuses to change course there is nothing I can do than to keep suggesting that using the branches/star puts additional potential for issues into that system. Sure if all the connections are great the reflection shouldn't be an issue, but then again if those CAN bus wires are moving around...
With the telephone style Jaguar CAN/serial connectors and a Jaguar say 15 feet away on an end-effector are you really sure the connector will not move at all so you can predict the characteristic of that network? Maybe on a bench test this would be okay but what about when that extended end-effector gets smacked by another robot?
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In your example of the wiring on a moving arm, I would be more concerned about an outright interruption in the connection (connector or wire breaking) than the changes in the characteristic impedance of the cables on the moving arm. My personal experience with moving wires around on such networks (while monitoring with a scope and using proper high-frequency probing techniques) is that the "lumps and bumps" move around in time a little but the overall peak amplitude and the decay time of the reflections are not affected in a significant way.
I may have overlooked them but I don't recall seeing strict limits on the wire size and the twists per foot for the CAN Bus wires to make the characteristic impedance of the wiring more uniform in any documentation from NI, CTRE or any other FRC related sources. I also don't recall seeing requirements for shielding of the CAN Bus wires. There is no requirement that the termination resistors are high accuracy types. Those are the sorts of things one would have to do to make the CAN Bus characteristics more stable but doing those things probably takes one past the point of diminishing returns and greater/unnecessary cost/effort.
With regard to risk management, please keep in mind that the size of the CAN Bus network in an FRC robot is a small fraction of the maximum size intended in the standard. Digital communication systems like the CAN Bus are developed with significant noise margins to guarantee successful implementations by relatively unskilled installers. That is where the restrictions such as the maximum length of the bus come from. As long as one is only using a fraction of the noise margin that has been built into the technology, one should be pretty certain of success.
Looking at the big picture, by following some fairly simple recommendations, the risk of the CAN Bus being the cause of a failure on the field is significantly less than other sources of risk such as choosing the wrong sort of drive train or scoring mechanism.