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

13 What is Stub Length? What are the Design Considerations Around it?

A stub is the electrical length of cable between a node’s terminal and connection to the CAN bus as shown in Figure 13-1. Since stub-lines are unterminated, signal reflections can develop in a stub that drives signal levels back and forth across a receiver's input thresholds, creating errors.

The ISO 11898-2 Standard specifies a maximum bus length of 40 meters, max speed of 1 Mbps, and maximum stub length of 0.3 meters. However with careful design, stub lengths can be longer. Below is a conservative rule of thumb to calculate maximum stub length with the idea that signal reflection due to a stub remains during transition time itself and dies down later. Designs with longer stubs than this may be possible with slower data rates and if the signal quality is acceptable in the network. We recommend system designers perform thorough testing with their network design to arrive at a conclusion.

CAN Network Showing Stubs Figure 13-1 CAN Network Showing Stubs
Equation 3. 2*propagation delay of signal in stub <= (1/3)* Rise or fall time of transceiver

For ISO1044: Fall time= 40 ns typical, say if x is stub length in meters, signal travels at 5 ns/meter in twisted pair cable.

Putting values in Equation 3, 2*x*5 ns/m <= (1/3)*40
x<= 1.33 meters.