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Can Connections

Practical Motorsport Wiring - Club Level

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I'm new to wiring, went through the courses, starting to wire in an elite 2500 in my rx7, but a little confused on how to wire the can connections. once the wire gets terminated, do i just loop it to the next termination?

The CAN bus is relatively straight forward and you just need to follow some basic guidelines. The bus needs to be a twisted pair with approximately one twist per 25 mm. If the bus is longer than 2 metres then you need a 120 ohm terminating resistor at each end (MoTeC diagram shows 100 ohm but in reality either 120 or 100 ohm works). You then add each node that needs access to the bus and make sure the length from the main trunk is less than 500 mm. The bus should look something like the picture attached.

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I’m installing a motec c127 into my e92 m3 when splicing into the PT CAN do I just do the splice as you guys have Described in the wiring fundamentals course?

Yes, you just splice into the CAN bus. Be careful if you are using the MoTeC harness that comes with the Race Kit, as the ECU CAN connections have terminations added in the harness, and if connecting to an existing properly terminated bus, you don't want to add additional CAN termination. Hopefully you can find some of the CAN wiring near the dash area (OBDII plug usually has it) to keep you length to the C127 as short as possible (spec is 500mm, but longer usually works)/

I’d have to check but I’m pretty sure there is a resistor right next to the OBD and I was going to use the motec harness in a case like this what would I need to do?

As you can see from the drawing posted above, there should only be two termination resistors on a CAN bus. So, you'll need to remove one.

With the vehicle power off, you can determine the bus termination by measuring the resistance between CAN Hi and CAN Lo signals. You should see about 60 ohms, if it's significantly less (30 ohms), then you have extra termination resistors installed and something needs to be removed.

thank you for the responses, its alot simpler than i expected.

Hi Guys

I have a full car setup im working on with canbus IR cameras and a lot of other electronics, minimum 10 CAN components in total. Is it ok to have 6+ items all all splicing off the same node in the trunk? and what happens if one of those items needs to be on a branch over 1m long?? So the layout is like a huge "T" Essentially all the incar electronics will need to come off the same place in the body harness and the trunk will loop around the rest of the car to the 4 x IR tyre cameras.


Ideally, you would daisy chain those together. So the bus goes from say your Front Left IR sensor (which has a terminating resistor) to the ECU to the Dash (or near the dash), and from there to you next CAN device (say the Front Right IR), then back to devices in the rear (Rear Right IR sensor, Rear Left IR, etc), finally you terminate the other end of the bus. Basically you have two CAN hi and two CAN low wires at each device connector, and you just chain them together. Often you can crimp both wires into the same terminal on the connector.

you have covered some of my question , I have a 4 metre long land speed bike , the M84 is at the back , the Plex “dash” is at the front , so i need a 1.5 metre( CAN) extension lead to join them . ( the plex lead is already 1+ metre) the Motec lead is 250/300 mm total lengthen is about 2.5 metres , so after all that , can i use a CAN extension lead to join the two ?

Steven, if your only two can devices are the M84 and the Plex dash (plus the USB-To-CAN (UTC) connection used for communicating with the M84), then you just need to connect the CAN bus between them using two wires twisted together about 1/2 to 1 twists per cm. I would use a 120 ohm resistor at the plex dash connector and another at whatever device is at the other end of the bus. Be aware that the UTC has a built-in terminating resistor so the bus is slightly over terminated when it's installed, but I would expect it to work. If you experience problems, consider just having one permanent termination resistor on the bus.

As a side note, how are you communicating with the M84 over CAN with the Plex unit? My understanding was that the M84 datastream is in a different (serial?) type format and not typical CAN format? I’m interested as I did a little research recently using 3rd party CAN logger with an M84 and the consensus seemed to be that it couldn’t be done.

The M84 does have a fixed CAN output stream based on the old Motec "data set 1", it is a bit odd in that it is a very long asynchronous frame with a CRC32 checksum on the end which is not very common in CAN comms. Many of the common dashes such as AEM, AIM, Plex, ECU master etc can handle it no problems but many of the less common or more basic devices only work with the far more common standard 8 byte CAN frames.

Thanks Adam.

So essentially it’s something the device developer needs to write firmware for over & above the common CAN protocol

It is still CAN so the electronic hardware required, wiring etc is all the same, just how they squeeze the data into the frame, the timing and the error checking is quite different. So yes its really just the software/decoding of it that is different. So most dashes that support it will have a special "Motec" setting since it is so odd.

Alex , glad others have answered your response , because it is more than my experience, i had help from the Australian agent i bought the Plex from , the plex 102pro is a nice little unit , and can log a lot —— given the M84 s limited logging

Cool, thanks for the clarification. I’m glad it wasn’t just me missing a trick haha

slightly obscure question but how do haltech black and white can harnesses work?

do the black connectors simply have a termination resistor in them?

I am trying to wire my motec PDM15 however I’m a little confused on the CAN portion. I am using the motec keypad and the UTC. I heard I could essentially run both outputs with the same CAN. Is this true? Also how do you wire the UTC?

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This is a picture of the UTC connector.

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On the Neutrik/Deltran connector, Pin 4 is CAN Lo, and Pin 5 is CAN Hi. The pin numbers are molded into the plastic of the connector.

The CAN connection is a bus, so you can connect multiple things together. There are rules to be followed, see the CAN & PC Connection appendix in your PDM manual.

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