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

2 Standard and Features

RS-485 is an electrical-only standard. In contrast to complete interface standards, which define the functional, mechanical, and electrical specifications, RS-485 only defines the electrical characteristics of drivers and receivers that could be used to implement a balanced multipoint transmission line.

This standard, however, is intended to be referenced by higher level standards, such as DL/T645, for example, which defines the communication protocol for electronic energy-meters in China, specifying RS-485 as the physical layer standard.

Key features of RS-485 are:

  • Balanced interface
  • Multipoint operation from a single 5-V supply
  • –7-V to +12-V bus common-mode range
  • Up to 32 unit loads
  • 10-Mbps maximum data rate (at 40 feet)
  • 4000-foot maximum cable length (at 100 kbps)