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| I believe what you are describing a transmission line's characteristic impedance which is affected by a number of things including the insulation type and size. When the CAN signals (or any electrical signal propagating along a transmission line) meets a change in impedance (note: end of the line is also a change in impedance) some or all of the signal reflects back to the source. A terminating resistor at the end of the cable that near matches the characteristic impedance of the transmission line reduces the reflections (in a perfect world none). 120 ohms has become the standard in the case of a CANbus. The resistive load also helps the two wires return to the recessive voltage faster after a bit is transmitted. This passive termination.
12V automotive power supplies are terribly noisy to begin with and the cables are not shielded so they pick-up noise from all kinds of radio signals. With so many different wiring harness chunks and connectors involved, the characteristic impedance across the communication system an be all over the place. In the real world, the signals don't look anything like those nice square shaped diagrams. So the active termination & bias circuits (TBC) were added to help deal with the very noisy CAN signals. In addition to trying to match impedance / prevent reflections on the cable, active terminators provide regulated power at the recessive voltage (2.5V), help filter out some noise, and suppress voltage spikes to protect the rest of the CAN devices' circuitry. Note, these active termination devices require a power source - usually a 5V supply, hence the 4 wire CANbus - 2 for the CAN High Low signals and 2 for power. ISOBUS actually specifies a independent power supply just for the TBCs so they have a clean supply all to their own. ISOBUS also specifies a quad twist cable instead of the usual twisted pair type wiring to help reduce noise on the rather long wiring harnesses on an implement (very long compared to the typical CAN system in a car)
In some cases, the signals are clean enough that the system can run without a terminator on boths, just one can get the job done. There could be lots of communication errors but the CAN protocol handles them and the messages get thru eventually . Some products build the termination into the device (which assumes it will always be installed at an end of the bus) so the external one on the wiring isn't needed.
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CANBus terminators... what are they anyway?
