SLLA486B May   2020  – October 2024 ISO1042 , ISO1042-Q1 , ISO1044 , ISO1050 , ISOW1044

 

  1.   1
  2.   Top Design Questions About Isolated CAN Bus Design
  3.   Trademarks
  4. 1 When Do I Need to Isolate CAN?
  5. 2 What are the Options Available to Isolate CAN Bus?
  6. 3 Now That I Have Isolated CAN Signal Path, How Do I Generate Isolated Power?
  7. 4 What’s the Reason Behind Terminating the Bus, Do I Need it, and How to Achieve it?
  8. 5 What’s the Difference Between Common Mode Range and Bus Standoff Mentioned in Data Sheet?
  9. 6 Now That I Have Taken Care of the Termination Resistor, What Other Components do I Need on the Bus Side?
  10. 7 When Connecting Isolated CAN Nodes in a Network, What Should be Done with the Floating Bus-Side Ground Connection?
  11. 8 Is There a Limitation on Minimum Data Rate That I Can Operate? What About the Maximum Data Rate Achievable in a Network?
  12. 9 Is There a Limit on Maximum Number of Nodes That I Can Connect in CAN Network?
  13. 10What Factors Decide the Maximum Communication Distance in a CAN Network?
  14. 11What is the Maximum Value of Bus Capacitance That Can be Introduced Between CANH to GND and CANL to GND? Can Higher Capacitance Damage the Device?
  15. 12Is There a Way to Extend the Maximum Communication Distance?
  16. 13What is Stub Length? What are the Design Considerations Around it?
  17. 14I am Seeing Larger Differential CAN Voltage for Some Bits of CAN Packet Compared to Rest of the Packet When I am Communicating in a Network with Multiple Nodes Connected. Why?
  18. 15References
  19. 16Revision History

8 Is There a Limitation on Minimum Data Rate That I Can Operate? What About the Maximum Data Rate Achievable in a Network?

Most isolated or non-isolated CAN transceivers have a protection feature called Dominant-Timeout (DTO). This feature disables the transmitter of a device if it holds the bus dominant for a time greater than DTO. This feature is useful in case of a software failure or a hardware failure that makes TXD low continuously. The CAN protocol does not allow transmission of more than 5 bits of same state in a row due to bit stuffing rules except in error condition. So in an error scenario, 5 dominant bits followed by 6 consecutive dominant bits of error frame needs to be transmitted. Hence, 11*bit time of one dominant bit <= DTO time. This decides minimum data rate (or maximum one bit dominant time period).

Though ISO1042, ISO1044, and ISOW1044 are able to support a maximum 5 Mbps data rate, the actual maximum achievable in a network is dependent on maximum cable length (for example, distance between farthest nodes), type of cable (which will decide signal speed in the interconnect medium), and total capacitance that exists across the CAN bus due to cable, individual nodes, PCB traces, connectors, and so forth. Bitwise arbitration is the key to CAN protocol. This means during arbitration phase of CAN packet, a bit sent by a transmitter needs to reach the farthest receiver and back to the transmitter which monitors via RXD for it to move to the subsequent bit in the CAN-ID part of a data packet. So the fastest bit time in arbitration phase has to be more than the loop delay of a transmitter node + 2*prop delay of cable (typically 5 ns/meter of CAT5e cable). This indicates there is an inverse relationship between maximum data rate in arbitration period and maximum communication distance. The maximum data rate during data-phase of CAN packet would be limited by the bit timing distortion introduced by the transceiver and by the controller's sampling point margin. Overall capacitance seen on the bus also impacts timing as dominant to recessive edge transitions may be elongated if a higher capacitance is seen on the bus.