Re: Talon SRX CAN Daisy-Chain Wiring
 

I understand it states "terminated at both ends".

Please read the Stub length section (4.8) of that specification, as it deals with branch length and signal propagation within a branch.

I'm, not stating that it is a great idea to have stub lines, all I was doing is questioning the "The termination resistor in the PDP only works if the PDP is at the end of the CAN bus" statement.

Re: Talon SRX CAN Daisy-Chain Wiring
 

I feel that some of the lessons that team members learn when participating in FRC is to balance risk, to prioritize tasks and to choose what challenges to undertake.

The CAN architecture has been developed to work a certain way with a proven tolerance of limited variations, like stubs up to a 0.3 meters * on the transmission line. When implemented according to the recommendations, it works very well. If someone reading this thread decides to take a chance and implement a non-standard CAN network architecture, they are assuming the risk that they may get degraded performance that will probably take a lot of time and energy to sort out.

It is not likely that many FRC teams will have the equipment (and the experience to use it properly) to trouble-shoot a CAN network that is suffering from phenomena such as reflections. For most teams, developing a robot to "play the game well" in the short time given is a challenge that pushes them to the limits of their abilities. Should teams also take on the challenge of getting a non-standard CAN network configuration to work reliably or should they follow the general recommendations regarding how to set up their CAN network and focus on the robot?

* 0.3 meters = 11.81 inches. This is just a bit longer than twice the length of the CAN Bus wires on the motor controllers (5.5 inches). Maybe CTRE is trying to save us from ourselves?

Re: Talon SRX CAN Daisy-Chain Wiring
 

Maybe. Though, I did see the board assembly without the housing at various offseason events when folks were asking for my opinion :)

Re: Talon SRX CAN Daisy-Chain Wiring
 

I should have said the CAN buss on a robot will act more like a 50 ohm buss with the two terminations so close together. Without the terminations at the ends of the buss, ringing, noise and other issues (especially when using unshielded wire) can affect your operation.

Re: Talon SRX CAN Daisy-Chain Wiring
 

The specification for CAN is 120 ohm (+/-12 ohms) terminating resistors. So when measured, the bus should be 54 to 66 ohms.

Re: Talon SRX CAN Daisy-Chain Wiring
 

Two terminating resistors 120 ohm each in parallel = 60 ohm The spec has varying values for the terminating resistors depending on the exact situation.

In answer to Dan's question. The termination resistors are suppose to be on the end to kill reflections. Canbus is actually very forgiving. We have one robot that has been missing one of its termination resistors for a couple of years. It is one of our primary demo bots & it has been working fine. BUT when I am troubleshooting issues, one of the first things I do (after making sure it is plugged in) is make sure everything is done according to specification. So if I was having comms problems, I would make sure the wiring is correct, termination resistors are place, value, & quantity. And then go beat on the programers. (and eat their corn dogs.)

Sorry for the me too

Re: Talon SRX CAN Daisy-Chain Wiring
 

If you CAN bus is suffering from reflections, the following can occur...
-DS will show Rx and Tx CAN errors intermittently. (Section 15 in Talon SRX Software Reference Manual).
-CAN bus utilization will be higher than normal. Since CAN error frames eat up bus time just like everything else, you'll notice the CAN utilization go up. This is helpful if you're in the happen of checking it every once in a while and knowing what is normal (kinda like gas-mileage in a car). This is also in section 15 of software ref manual.
-All CAN nodes will blip red intermittently during normal use. Basically the same symptom as Section 16.4. Both common-id talons and bad signal reflection can cause frames to have bad overshoots/undershoots, causing frame CRC checks to fail which causes CAN devices to transmit error frames, which you will see when the Talons/PDP/PCMs blip red during normal use.

One way to reproduce a similar symptom (just to see what the red blip looks like) is to deliberately leave a couple common-ID talons on your can bus. Then power up your robot (leave disabled). All your talons will rail-road orange (healthy CAN bus and disabled). Now sit and watch the Talons for a minute or so. They will blip red every once in a while when the two common Talons inevitably steps on each other's arbid.

Re: Talon SRX CAN Daisy-Chain Wiring
 

*The characteristic impedance of the cable cannot be measured with an ohmmeter.

Re: Talon SRX CAN Daisy-Chain Wiring
 

Are you willing to take a chance on those "errors" popping up in a finals match?

Students - please take the advice of Ether and others and use CANBus as intended. Terminate at both ends (done for us at one end on the roboRio) only, keep stubs under .3m and total length under 40m. Use shielded twisted pair when possible. The standards exist for a reason - much testing was done to arrive at the numbers in the standards and the designers of the CANBus nodes (on the roboRio, PDP and TalonSRX for example) follow the standards in their designs. Intentional deviation from the standard puts your design in unknown territory probably without the time or facilities to do proper testing. This is most unwise.

HTH

Re: Talon SRX CAN Daisy-Chain Wiring
 

Interesting story here. Belden had made and marketed 8451 shielded twisted pair as a 600 ohm audio cable and everyone used it in terminated 600 ohm audio applications for years. Many guys in remote TV production noticed that it behaved oddly in long runs, over 1000 ft. So ABC did some testing and determined that the impedance of the cable was actually more like 120 ohms. The cable had originally been intended to replace standard telephone wiring which is 19 gauge unshielded twisted pair and always terminated in 600 ohms. Audio wiring and equipment had been designed for using a terminated 600 ohm line until this became known. So in an attempt to modify designs, many manufacturers started using a low impedance source or line driver. Benchmark Audio did some extensive testing and decided on a 30 ohm source impedance using this cable. That is the typical equipment output these days for analog audio.
Please remember that while the wiring is intended to act as a transmission line, termination is important for noise immunity and signal level in our application. Even at 1 Mb/s data rates, a few feet does not begin to exhibit true transmission line characteristics. A half wave length is approaching 500 feet at that frequency.
When I mentioned the problems that teams have had in the past with terminations, most of the problems were related to poor soldering, using resistors other than 120 ohms, insulation and other quality control issues.

Re: Talon SRX CAN Daisy-Chain Wiring
 

Re-quoted for truth.

CAN is somewhat forgiving, but why make it operate at the bleeding edge of stability when there's no good reason to do so? If you knew what you were doing, you'd terminate the ends as recommended, so why not just do that?

Re: Talon SRX CAN Daisy-Chain Wiring
 

Thanks for clarifying. In the end, we ended up terminating with the PDB.

Re: Talon SRX CAN Daisy-Chain Wiring
 

Just as the designers intended! Well done! :D