@alan707 Because if you look at the diagram you posted, in order to build a full network you should terminate on both ends. If you were making a dongle (and not a full bus) you would only need one 120 ohm for your device.

My understanding was that as long as you have a single terminator resistance, only 120 ohm resistance is needed (especially for very short lengths of wires like the ODB GW). I've built CAN buses before with two Carloops on each and and a single 120 ohm resistor and it worked. As I said before, not critical and 240 ohms will work just as well, just wanted to understand the reasoning.

Two 120 Ohm resistors or one 60 Ohm resistor should always be used. 240 Ohms is a mistake (See #7).

The CAN resistors serve two purposes:

1. To absorb reflections at the ends of the transmission line (bus wires).
   For this purpose, a resistor should be added at each end of the transmission line, and the resistance should match the characteristic impedance of the transmission line, which is 120 Ohms (regardless of cable length, assuming the correct twisted pair cable is used).
   For very short transmission lines (as is the case here), reflections are irrelevant, so termination resistors are not needed to absorb reflections.

2. To pull together the voltages on the bus wires after the transmission of a dominant bit.
   HS-CAN uses differential voltage signaling, where a voltage difference indicates a dominant bit (0), and no voltage difference indicates a recessive bit (1). HS-CAN nodes actively induce a voltage difference to transmit a dominant bit, but do nothing to "transmit" a recessive bit (they rely on the termination resistors to return the bus to the recessive state after the transmission of a dominant bit).
   For this purpose, one or more resistors should be distributed along the transmission line, with a low enough total resistance to overcome the capacitance of the transmission line and nodes within the bit transition timing requirements, but a high enough total resistance to ensure that nodes can overcome the resistance to induce the necessary voltage difference to transmit a dominant bit.
   For very short transmission lines (as is the case here), the capacitance of the transmission line is small, so a higher than normal total resistance can technically be used.
   However, since all HS-can devices are designed to be able to overcome 60 Ohms (two parallel 120 Ohm resistors) when transmitting a dominant bit, there is no need to deviate from that resistance value, and doing so risks having insufficient resistance to overcome the capacitance of the system within the bit transition timing requirements.