Can I branch the CAN to go to two separate places?

[philso]

Originally Posted by wireties

1Mb/s but the actual throughput may vary. CANbus uses NRZ encoding and bit stuffing etc so realized throughput is usually in the 850kb/s range.

one implies the other (but not really sure what you mean by edge rate)

Where does this 1 Mbps figure come from?

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.

Data-rate is a measure (bits per second) of the instantaneous rate at which the data is transmitted over the bus, when there is data. One can also measure the average data-rate taking into account the time when the bus is inactive (unused) and any error correction made necessary due to noise or other problems (resending of packets).

Edge-rate is a measure (volts per microsecond) of how fast the high-to-low and low-to-high state transitions take place. It is the high frequency energy contained in fast transitions that causes reflections. It is possible to have a CAN bus system with a low data-rate (say 1 bit per second) that has significant reflections because there are impedance mismatches AND the edge-rate is very high (the parts I am using have a spec of 35 nsec. minimum).

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.

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. As long as the reflections after a bit transition has died down by the middle of the bit period, the detected state of the bit will be accurate. This technique was developed a long time ago to make it easier for communication systems like CAN Bus to minimize the effects of reflections in the system.

Many of the CAN transceiver chips can be set to produce lower edge-rates, often using an external resistor. This allows the system/circuit designer to reduce the energy in the reflections (and hence their peak voltage and duration) in the system based on his/her knowledge of the length of the transmission lines (bus length), the amount of impedance mismatch expected and the maximum data-rate required.

The long and the short of it is that the CAN Bus standards were set up with the ability to tolerate some amount of reflections since the people developing the standards understood that the world is not perfect and that the systems will not be manufactured and installed perfectly. 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.

[wireties]

Originally Posted by ratdude747

QFT. Any time termination matters (such as CAN), you've entered the wonderful world of RF where reflections and the like are real. In RF you get to throw away everything you know about electricity and circuits, aside from Kirchoff's laws. Ohm's law? Kiss it goodbye, it doesn't apply any more. Wire spacing and twist rate do matter a ton.

All the normal rules still apply but there are additional rules (terms in the equations). You are looking at the complex impedance of the load (which did not factor in at low frequencies) rather than simple resistance. The size and shape of the wire and connector pins all matter because they help determine the reactance portion of the impedance (resistance plus reactance). Balancing the impedance of the source and the load become important to reduce mismatches that can cause ringing (reflected waves bouncing around willy-nilly).

So for CANbus the sources have a particular output impedance that performs best when the load has a matching impedance (same resistance and opposite reactance). To keep things simple use wire much like the green/yellow that comes with the Talons. Use connectors with cross sections close to the cross section of the wire. Wire things in series from source to load (like CTE recommends and the CANbus standards dictate). Use a 100-ish ohm resistor for the load (termination). To do otherwise is to ask for troubles that are very difficult to diagnose.

[jee7s]

Originally Posted by wireties

All the normal rules still apply but there are additional rules (terms in the equations).

This is taking the thread a tad off topic, but, I respectfully disagree. It's not that there are additional rules. The rules are the same but you can't use as simple a model of the rules.

The so-called "normal" rules are really a number of simplifications and assumptions that apply at DC that don't apply when there is change in the signal. Even the complex impedance is a simplification that applies to lumped element models. Fundamentally, both of these are case specific subsets/simplifications of Maxwell's Equations, which are the set of laws that govern electromagnetism at non-relativistic conditions.

I say this because I think it further illustrates the bad thinking I highlighted earlier: one must acknowledge the assumptions that are used (like assuming conductors are ideal) before applying a concept. It's one thing to use a suitable simplification because a set of conditions are met. I don't think any engineering would happen if we needed to do vector calculus when determining a load instead of using V=IR. But, as a good engineer, you need to know when you can simplify the model and when you cannot.

As an aside, @ratdude: even Kirchhoff's Laws don't always apply, particularly if a changing magnetic flux is present.

[wireties]

Originally Posted by jee7s

This is taking the thread a tad off topic, but, I respectfully disagree. It's not that there are additional rules. The rules are the same but you can't use as simple a model of the rules.

Thus the "terms in the equations" comment ...

[Arhowk]

Originally Posted by dirtbikerxz

just run a really long strip. No point in introducing points of failure into an already relatively "fragile" system.

From both a CSA and a head electrical's perspective these seem like two conflicting points. I've seen a great deal of wire cut because of things like drilling, nuts/bolts, zip ties, or just normal wear and tear (robot on robot action). Resilience does not favor one or the other in an "all encompassing" scenario.

[jnicho15]

Early in the season, we tried using a central distribution panel for our SRXs in order to decrease the chance of cascading failures. Although this should work since CAN is wired in parallel, we had many problems especially since we were using "PWM" connectors to each one. We then switched to a soldered chain and had fewer problems.

[jee7s]

Originally Posted by jnicho15

Early in the season, we tried using a central distribution panel for our SRXs in order to decrease the chance of cascading failures. Although this should work since CAN is wired in parallel, we had many problems especially since we were using "PWM" connectors to each one. We then switched to a soldered chain and had fewer problems.

Emphasis above is mine.

For the record, CAN is "wired in parallel" if and only if you regard the conductors as ideal. The real world scenario has imperfections like resistance, inductance, and capacitance, that make for a much more complicated electrical reality than "wired in parallel". Particularly at 1Mb/s, those lumped elements and the aforemnetioned reflection problem mean that the CAN topology is best implemented as a terminated bus with short branches. The PWM connectors are not the issue, and many teams use that style of connector in a canonical implementation of CAN with no issue. Go back to the Jaguar and the CAN bus uses RJ style connectors with no issues if the connectors are properly assembled and inserted. Connectors in CAN are perfectly acceptable, it's the network topology that needs to be respected.

In other words, like Alan and others have said, don't tempt fate. Wire CAN as recommended in the documentation for best results.