You mean the docs posted above? The ones that say it's not terminated and supposed to be connected to a properly terminated bus? After I roughly explained that it probably doesn't matter in some circumstances?

Yeah, it probably has a resistor built in...

 

Direct from the X-series manual...

The X-Series controller does not have any internal termination and is intended to be connected to a pre-existing, properly terminated network.

After I posted I looked further into that terminated resistor and it says minimum of 4W, between 110-130 Ohm. I'll dig through my spare parts and maybe stop at the local Axe Man surplus store with my multimeter if I don't find what I need in my small box. Thanks Deezum for clarifying that it doesn't matter if the controller sits outside the resistor, that was my main question.

 

4W is overkill
1/4W

 

0That's interesting Deezums, I recall seeing there was no resistor in the gauge, which makes sense if you were setting up a CAN network and not just a single CAN gauge, however I would lean towards the MS3 having one built in, though that's pure conjecture on my part. One last data point I'll add is I didn't trim down the harness at all on my gauge, so the RAN run in my car is a pair of ~6-8' wires. From what you're saying, I would expect there to be issues if there weren't a resistor on the CAN at some point, which leads me to my assumption that there is one in the ECU.

 

Curious, did you either not read all of what I posted or did you just not comprehend? Because literally everything you ask or infer has already been stated.

MS3 has it built in, it's R8 on a MS3x board. That's not enough, as I showed in pictures a few posts above. Does it matter? I also covered that above.

 

Just didn't comprehend. I'll shut up now and lurk some more. As you said, it works for me, so I'll leave it alone. Keeping all the above in mind though if ever I encounter issues.

 

Yep, you’re correct @deezums , no resistor in the AEM gauge, it took me a while to find the terminator plug they sell. Then they have cables etc. It is easier and I imagine just as effective to add a second 120 ohm resistor to meet the spec for high speed can. Interesting enough is that there is a lower speed can spec using only one resistor. Can bus units can be multi dropped and then compete for the bus, which probably increases the importance for the second resistor to pull the signal back to zero quickly. We are getting away with using only the ms3 resistor probably because of using slave mode and only point to point, not multi drop. I’m going to see about adding a resistor the next time I plan to pull the ecu out and certainly if another can sensor is added. I think I just gave it a shot originally and was happy with the unit reporting no can errors.

 

Low-speed CAN actually involves even MORE termination resistors: it requires each device to be internally terminated with two resistors, and that's very much irrelevant to everything here since the AEM gauge runs at 500kb/s.

It's true that the CAN resistors also help the open-collector transceiver outputs return to zero, and it's also true that going from 0->1 resistor is a much bigger improvement than going from 1->2 for draining the bus capacitance. However, whether or not the transceivers are competing (rather than one sending and the others listening silently) for the bus is basically irrelevant to how well a single resistor works to drain the bus.

For the other purpose of absorbing reflected signals discussed ad nauseam in this thread: all else being equal, it's better to have the parallel terminating resistor closer to the receiver.

As @deezums has posted approximately 400,000 times in this thread: nobody can tell you whether your CAN bus will work with one resistor. If it does work, it's because the cables are short (500kb/s allows a max length of 100 meters vs. however long your cable is), meaning capacitance is lower and reflections are closer in time and finish before the signal is sampled. It's kind of like "getting away with not using fuses."

 

@afm Thank you for adding to the conversation. I've not had a need until now to dig into CAN and this is quite informative. I've just pulled up a great bit of information in a TI blog and I think it agrees with all you've posted along with some additional info on split termination which might be even better. the-importance-of-termination-networks-in-can-transceivers The other factor he mentions that the resistors need to be able to handle a potential power fault on the bus, such as supply voltage shorting to CANH. That might require a higher wattage resistor. The cable supplied with my AEM gauge is about a meter long and from what I am reading, the best place to put the second terminator would be to cut into the cable and place it close to the connector which plugs into the gauge. Otherwise I will end up with terminator in the MS3, one about 12 inches outside of it, and a run of 1 meter up to the second transceiver. Since I will more that likely keep the AEM gauge in possible future configurations cutting it's cable up would still be ok, I would keep it as the other 'end' of the BUS. @irodd what did you end up doing?

 

I have the termination resistor installed on AEM controller side since 2018.
It took 5 cents and 5 minutes.