SLLA486A May   2020  – May 2021 ISO1042 , ISO1042-Q1 , ISO1044 , ISO1050 , ISOW1044

 

  1.   Trademarks
  2. 1 When Do I Need to Isolate CAN?
  3. 2 What are the Options Available to Isolate CAN Bus?
  4. 3 Now That I Have Isolated CAN Signal Path, How Do I Generate Isolated Power?
  5. 4 What’s the Reason Behind Terminating the Bus, Do I Need it, and How to Achieve it?
  6. 5 What’s the Difference Between Common Mode Range and Bus Standoff Mentioned in Data Sheet?
  7. 6 Now That I Have Taken Care of the Termination Resistor, What Other Components do I Need on the Bus Side?
  8. 7 When Connecting Isolated CAN Nodes in a Network, What Should be Done with the Floating Bus-Side Ground Connection?
  9. 8 Is There a Limitation on Minimum Data Rate That I Can Operate? What About the Maximum Data Rate Achievable in a Network?
  10. 9 Is There a Limit on Maximum Number of Nodes That I Can Connect in CAN Network?
  11. 10What Factors Decide the Maximum Communication Distance in a CAN Network?
  12. 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?
  13. 12Is There a Way to Extend the Maximum Communication Distance?
  14. 13What is Stub Length? What are the Design Considerations Around it?
  15. 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?
  16. 15References
  17. 16Revision History

9 Is There a Limit on Maximum Number of Nodes That I Can Connect in CAN Network?

Each node presents a certain differential load across the CAN bus which is specified as RID (differential input resistance) in the Receiver Electrical Characteristics section of the device data sheet. For ISO1044 and ISOW1044 is specified at minimum of 40 kohm. Other than the two 120 ohm termination resistors on far ends of the network, each node’s differential resistance combines in parallel to load the transmitter that is about to drive dominant on the bus. The equivalent parallel resistance that a driver should see needs to be more than 45 ohm because 45 ohm is the minimum load a driver is specified to drive and produce a minimum differential voltage of 1.4 V (as specified in Driver Electrical Characteristics). So in the case of ISO1044 and ISOW1044, if we connect 222 nodes on the bus, the equivalent differential resistance all CAN nodes offer on the bus is 40000/222 = 180 ohm. This 180 ohm in parallel to two 120 ohm termination resistors gives a 45 ohm equivalent loading on the driver. This is the theoretical limit on the maximum number of nodes on a CAN bus. Practical system aspects will limit this further.