SLLA272D February   2008  – May 2021

 

  1.   Trademarks
  2. 1 Introduction
  3. 2 Standard and Features
  4. 3 Network Topology
  5. 4 Signal Levels
  6. 5 Cable Type
  7. 6 Bus Termination and Stub Length
  8. 7 Failsafe
  9. 8 Bus Loading
  10. 9 Data Rate Versus Bus Length
  11. 10Minimum Node Spacing
  12. 11Grounding and Isolation
  13. 12Conclusion
    1. 12.1 References
  14. 13Revision History

8 Bus Loading

Because a driver's output depends on the current it must supply into a load, adding transceivers and failsafe circuits to the bus increases the total load current required. To estimate the maximum number of bus loads possible, RS-485 specifies a hypothetical term of a unit load (UL), which represents a load impedance of approximately 12 kW. Standard-compliant drivers must be able to drive 32 of these unit loads. Today’s transceivers often provide reduced unit loading, such as 1/8 UL, thus allowing the connection of up to 256 transceivers on the bus.

Because failsafe biasing contributes up to 20 unit loads of bus loading, the maximum number of transceivers, N, is reduced to:

Equation 3. GUID-EBB4A378-5FB0-4A78-A52F-0CAC49329AC6-low.gif

Thus, when using 1/8-UL transceivers, it is possible to connect up to a maximum of 96 devices to the bus.