Quote:
Originally Posted by ratdude747
I was taught in electromagnetics class that Kirchoff's laws do apply in RF, and specifically those alone are what do translate from circuit theory to RF unchanged. While a changing flux can cause all sorts of fun voltages, in a pure snapshot of an RF system all voltages will be accounted for someplace. It may be someplace you don't expect, but it will be accounted for.
Otherwise, obviously I agree.
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I agree. In any active AC circuit at a given instant in time Kirchoff's Law ought to work. Now over time, that's a whole different matter. Once one starts decoupling the voltage and the current over time (phase) with capacitance and inductance that transition makes interesting things happen like resonance. Then we gotta start pulling in math like Fourier transforms and Laplace transforms.
Since CAN signals are NRZ digital (with the 1&0 voltages dependent on the PHY layer design) they represent a typical digital square wave of odd-integer harmonics with differential signal. The issue, and the TDR neatly demonstrates this concept, is that an analysis over time (not at any given instant) can produce reflections when the termination or legs (even the legs of the circuit inside the devices) get too long. So it's not merely the real component of the terminator resistance at work. It's the capacitance, inductance and impedance of the circuit as a whole. (Active AC components tend to introduce mathematical imaginary or polar numbers into circuit analysis so when I say real resistance I mean DC resistance.)
Long story short - I don't think anyone in the last few posts disagrees that students shouldn't follow the guidelines. No one wants to have intermittent phantom problems that only happen when a certain pattern of traffic is sent over that CAN bus. I don't think I've ever seen a FRC team test their CAN bus with a
BERD/BERT even though the speed is pretty high on that circuit (a T1 digital telephone circuit is basically 1.544Mbps and these are usually tested with something like a
T-BERD). There's limited rational reason in the last few years of robots that the CAN bus configuration as recommended can't be achieved. Even if you somehow put a Talon 14 feet up an end-effector you could avoid doubling back with the CAN buss wire (which given you'd be running the power up there seems sort of strange) by simply ending the buss up there.
There are some good examples of why a star configuration has advantages if you suspect your devices might fail to be connected. However, like in computer networking, how far does one want to go to get a star network without the circuit implications? Active
CAN hubs? How about a
CAN switch? (NOTE: I am making no suggesting the 2 links I provided are FRC legal or even recommended, just pointing out that such devices exist with evidence.)