CAN Higher Layer Protocols Q & A

Questions & Answers

Q: In introduction, you show Canbus as a 2 wire network. It is my understanding this is not exactly true.. All devices MUST share common ground. The Standard Hardware receiver are not differential in nature.
A: You are correct, it is recommended to have a ground, but not always needed

Q: I Can open a deferential protocol as well...?
A: If you mean CANopen, it is a higher layer protocol, which uses CAN as Datalink layer. So on the physical layer it uses CAN and this is differential.

Q: Is there any low level CRC check that is not on the 8 bytes of data.
A: Yes, in the Datalink Layer of CAN there is a CRC which is checked in hardware for every frame.

Q: Is there any low level CRC check that is not on the 8 bytes of data.
A: The CRC is not part of the payload (the 8 data bytes), it has a extra field in the frame.

Q: Are all CAN open IDs 11 bit?
A: In CANopen it uses 11bit-IDs by default, but 29bit-IDs can be used. This is then part of the configuration for the services. But CANopen itself could exist in the the 11bit range.

Q: How CRC detect error
A: Its done in hardware. Every transmitter submits a crc in the crc-field, and the receiver checks if this is correct. All done in hardware/lower layer, so you won’t see it. If the crc is correct then the hardware acceppts the frame and passes it to the micro.

Q: Are j1939 PGNs "controlled"? as in does a vendor have to register the PGN? or is it "open"?
A: PGNs are are predefined in the DigitialAnnex, but c of course could be changed. So you would say it’s open, but the plud-n-play get lost if you don’t follow the recommendation.

Q: What do you recommend as your best source of SPN/PGNs identification ...?
A: Usually the DigitalAnnex. This is a list with all predefined PGNs/SPNs. And the docs of the device itself ;)

Q: why trucks using j1939 and not iso15765
A: if you mean iso15765-2, it is only a transport protocol and says nothing about the content which is transported. In CANopen and J1939 even the content is defined somehow. ISO-TP (iso15765) knows only binary blobs and the transported data has to be interpreted later (like in UDS)

Q: If I have a list of PGNs but cannot identify them, what source would you recommend?
A: Digital Annex from the SAE or device spec/docs.

Q: Is it possible to have J1939 and CAN Open devices, (thus two communication protocols) on the same bus?
A: Yes in general, if the CAN-IDs don’t collide. So if you use CANopen with 11bit IDs both can run on the same bus without problems.

Q: What is the biggest concern between mixing protocols like Canopen and j1939
A: Mixing CAN-IDs.

Q: Digital annex is only available from sae and not free?
A: Yes, the Annex is not free, some tools have interpretation

Q: Does message identifier also identifies the device to which the data is sent?
A: Depends on the protocol, and the service.

Q: Could you tell what kind of protocol the KOMATSU machines work
A: KOMATSU drills use J1939 and CAN Open protocols.

Q: What is the best way to get pricing on emotas software tools like CANopen DeviceExplorer?
A: For North America you can check out gridconnect.com or contact us. For everywhere else, please contact emotas directly.

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