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ISO1042: Some questions

Part Number: ISO1042
Other Parts Discussed in Thread: , TCAN332

Hi Sirs,

Sorry to bother you

We have some question on CAN Transceiver 

1. Can different CAN Transceivers be able to communicate in tandem?
2. What is the maximum number of serial connections allowed by CAN Transceiver?
3. What is the maximum distance allowed by CAN Bus?
4. We expects that through the structure of the following figure, can the connection be established?

  • Hi Shu-Cheng Lin,

    Thanks for asking your question on E2E! These are some great questions. A lot of these questions have related answers - so If you don't mind I'll combine your questions. 

    How many CAN transceivers can work on a single bus? (1) What is the maximum bus length? (3)

    A: This depends on a few different factors. The ISO1042 theoretically supports 100 transceivers on a single bus segment. But how many can you do on a practical network? And how far apart can the transceivers be spaced apart? 

    The key variables we need to consider here are data rate, cable length, and parasitic loading of the bus. 

    Data rate: the faster the data rate, the more the parasitic loading and cable length could cause errors. If we want a really long bus with a lot of transceivers we need to be using a slower data rate. The ISO1042 supports 5Mbps CAN FD, but the TCAN332DCNT only supports up to 1Mbps. So I'll assume the max data rate for your system is 1Mbps. If you are having trouble with your network - one easy thing to try would be to reduce the data rate. 

    Cable Length: The ISO 11898-2 Standard specifies a maximum bus length of 40 m and maximum stub length of 0.3m. The stubs are the length from the horizontal bus line you have drawn above. However, with careful consideration you can exceed this bus length by trading off number of nodes or lowering the data rate. 

    Parasitic loading: Each CAN transceiver you add to the bus will add some capacitance. For example, if you look at ISO1042 you can see CI and CID defined for the receiver electrical characteristics. For each ISO1042 added to the bus you are adding ~30pF to the bus. But this is likely not the whole picture! Many times discrete capacitors are added to CANH and CANL for EMC considerations (and these would need to be counted). TVS diodes can be added to protect each transceiver and these will add capacitance to the bus (typically defined as junction capacitance). Also common mode chokes are sometimes added in series with the CAN lines for EMC reasons and this will also add capacitance. Sometimes even the connectors to the bus can add some capacitance - and all of this needs to be taken into account. So both the number of CAN transceiver needs to be taken into account, but also the components surrounding each CAN transceiver. 

    So I've defined the key parameters - but I haven't really answered your questions. And that's because these questions are hard to answer. But there are a couple solutions to get you started. For example, CANopen network design guidelines allow the network to be up to 1 km with changes to the termination resistance, cabling, less than 64 nodes and a significantly lowered data rate. 

    I also have some test data:

    C&S tested our ISO1042-Q1 part on a homogeneous network with 16 nodes and a heterogeneous network with 16 nodes. Both of these networks were able to pass C&S's interoperability test specification for high-speed CAN transceivers at 5Mbps.  

    I developed a isolated CAN demo with 6 nodes and 30m of cabling. It was able to run several days error free at 5Mbps.

    You can find some more information about this in the 9.2.2.1 Bus Loading, Length, and Number of Nodes section of the ISO1042 datasheet (page 25)

    What is the maximum number of serial connections allowed by CAN transceiver? (2)

    A: I am not sure what you mean by this. CAN is a bus connection - so I am not sure what you mean by serial connection. Can you clarify the question for me? 

    Do I expect that the structure you have drawn in the figure would work? (4)

    A: Your figure has 10 transceivers. I will assume that stub length is kept to a minimum and the other system components around the transceiver do not add to much bus capacitance. My expectation is this network would work be able to work with a bus length of less than 40m at 1Mbps. 

    My apologies for the length response. Please let me know if anything doesn't make sense or you need me to clarify my points. Hope this helps! Please let me know if I've answered your questions. 

    Best regards, 

    Dan

  • Hi Sirs,

    I have a question about TI/ISO1042BDWV and TI/TCAN332DCNT.
    It is currently expected to place these two sets of CAN Transceivers on different boards because ISO1042BDWV has GND isolation.
    How to cut GND when connecting with TCAN332DCNT CAN Bus? Is there any precautions for reference?
    Thank you!

  • Hi Shu-Cheng Lin,

    I'm not sure I understand your question. Would you be able to provide a block diagram of your system?

    ISO1042 and TCAN332 will be able with each other over CAN if you connect CANH and CANL with correct bus termination. They do not need to have the same GND.

    Best regards,
    Dan
  • Hi Sirs,

    Our function block as below

  • Hi Shu-Cheng Lin,

    ISO1042 should be able to communicate with TCAN332 in this type of block diagram. This is a similar system to an automotive demo I created last year. The bus termination is not shown in this diagram. Just ensure that there are 120 ohm termination resistors on the furthest nodes on the network. 

    Best regards, 

    Dan

  • Hi Sirs,

    Thanks for your reply.

    Update our question as below

    Our function new function block as below

    1. When ISO1042 is connected to TCAN332 as shown below, should the GND of TCAN332 be connected to ISO GND or Digital GND of ISO1042?
         Is it only necessary to connect to one of the GNDs? Are the two GNDs separated by capacitors? Please tell us how to connect them?
    2. When ISO1042 is connected to TCAN332 as shown below, CANH/CANL of ISO1042 output is 5V level. Is there any problem if TCAN332 is 3.3V level?

    3. When ISO1042 is connected to ISO1042 as shown below, does ISO GND and Digital GND need to be separated by capacitor?
    4. When ISO1042 is connected to ISO1042 as shown below, because it is divided into two different boards, is ISO GND connected to ISO GND? Is VCC2 connected to VCC2?
         If we add one +5V to VCC2 on one of the boards, let the two VCC2s have different +5V, but ISO GND is connected. Is it feasible?

    5. If our architecture uses cable connection separately, does the CAN Bus signal need to be impedance controlled on the cable? How many ohm impedances should be controlled?
    6. The Termination 120-Ω resistor is seen in the ISO1042 and TCAN332 specs. How do I place the Termination resistor according to our architecture?

    7. Does CANH/CANL on the PCB trace need to be impedance controlled? How many ohm impedances do you need to control?

  • Hi Shu-Cheng Lin,
    Those are some really good questions and I want to make sure I answer them thoroughly. Please allow me until the end of the day tomorrow to put together a response to your questions.
    Best regards,
    Dan
  • Hi Shu-Cheng, 

    Thank for including an updated function block diagram. It is really helpful to our conversation. Please see answers below:

    1. When ISO1042 is connected to TCAN332 as shown below, should the GND of TCAN332 be connected to ISO GND or Digital GND of ISO1042?
         Is it only necessary to connect to one of the GNDs? Are the two GNDs separated by capacitors? Please tell us how to connect them?

    Basically, all of the GNDs on the CAN bus should be at the same potential. I've made a simplified version of your system to explain this. In the simplified system, there are two isolated CAN transceivers and three non-isolated CAN transceivers. 

    GND_A is used for everything in relation to CANH and CANL. So for ISO1042, this means GND2 and for TCAN332 this is the only GND. Note that Vcc_A (which is 5V) and Vcc_B (which is 3.3V) are both with respect to GND_A. 

    GND_C is for everything connected on side 1 of ISO1042. So this would be shared by the micro-controller and the other circuitry on side 1 of the device. Note that Vcc_C is 5V with respect to GND_C. 

    There should be no path from GND_A and GND_C. No capacitor is needed to separate them. 

    This isn't the only way to do this though! We can actually isolate each ISO1042 node from each other as well. This would look like this:

    In this case the system with MCU_A is isolated from both the non-isolated transceivers (referenced to GND_A) as well as the system with MCU_B (referenced to GND_D). For this system there should be no path from GND_C to GND_D either. 

    2. When ISO1042 is connected to TCAN332 as shown below, CANH/CANL of ISO1042 output is 5V level. Is there any problem if TCAN332 is 3.3V level?

    3.3V CAN systems are inter-operable with 5V CAN Systems, so yes this will be fine. No special considerations are needed. 3.3V CAN dominant and recessive states are valid for 5V CAN and vice versa. This question is answered rather well in the application note linked here.

    3. When ISO1042 is connected to ISO1042 as shown below, does ISO GND and Digital GND need to be separated by capacitor?

    I think I answered this in the above questions, but no capacitor is needed to separate ISO GND from digital GND. 

    4. When ISO1042 is connected to ISO1042 as shown below, because it is divided into two different boards, is ISO GND connected to ISO GND? Is VCC2 connected to VCC2?
     If we add one +5V to VCC2 on one of the boards, let the two VCC2s have different +5V, but ISO GND is connected. Is it feasible?

    As I explained in my simplified block diagrams above, either would be acceptable.

    If you want these boards to be isolated from each other, then VCC2 and GND2 of one board should not be connected to VCC2 and GND2 of another board. (Second block diagram I uploaded in this post)

    If they only need to be isolated from the rest of the network, they can share VCC2 and GND2. (First block diagram I uploaded in this post)

    5. If our architecture uses cable connection separately, does the CAN Bus signal need to be impedance controlled on the cable? How many ohm impedances should be controlled?

    ISO 11898 requires a cable with nominal impedance of 120Ω. The termination resistor should match the nominal impedance of the cable. So both the termination resistor and the nominal impedance of the cable should be 120Ω. 

    6. The Termination 120-Ω resistor is seen in the ISO1042 and TCAN332 specs. How do I place the Termination resistor according to our architecture?

    The termination resistors should be placed on the furthest nodes in the network. Assuming the ATX 12 Carrier board doesn’t have a substantially longer cable, I would think that the termination resistor should be close to the BCM in the lower left hand corner and the LED driver on the upper right hand side of your block diagram. 

    7. Does CANH/CANL on the PCB trace need to be impedance controlled? How many ohm impedances do you need to control?

    CANH and CANL should be laid out as symmetrically as possible on the PCB. You want them both to be good thick traces and have the same length and impedance. Having a shorter distance between the connector on the board and the cable is preferable. As to the impedance of the actual trace, this isn't of too high of concern. CAN is a relatively slower speed protocol, so controlling the impedance on the PCB to a specific number isn't really going to change performance in a measurable way. The cables will have a much larger impact. 

    Please let me know if I've answered all of your questions and if you need me to clarify anything! 

    Best regards, 

    Dan