Hi all, I recently stumbled upon this project and I love the work you’re doing here. I just performed my first read-through of the UAVCAN Standard V1.0-alpha, so I might not fully grasp all the details (yet). Anyway, one thing I noticed is that there’s no mention of the required cable impedance in the Physical Layer chapter. There is a nice reference to the ISO11898-2 standard, which does specify the required impedance (as far as I know). My question is whether you’d think it would be a good idea to include some statement about this in the UAVCAN Standard.
The reason for my question is that the referenced CiA 103 is quite specific about this:
The characteristic impedance of the cable shall be 120 Ω; ±10 % in the frequency range from DC up to 500 kHz.
A fact that may complicate matters is that for short cable-runs, the characteristic impedance of the cable is not as important (in my experience). So users may be tempted to ignore this requirement since it “already works”. Also, controlled-impedance cables are generally quite expensive compared to a simple twisted-pair. Also, I’m not sure about the availability of controlled-impedance cables with “Micro-connectors”.
A common solution (I’ve seen) for budget CANBus systems is the use of RJ45 connectors combined with standard Cat5 Ethernet cables. It turns out that these cables already have a spec on characteristic impedance, but 100 Ohm instead of 120 Ohm. In this case, a pair of 100 Ohm termination resistors could be used. As you know, these types of cables/connectors are very cheap and widely available.
Some general sources about ISO11898-2 which contain some insight into the design-considerations: