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

5 Cable Type

RS-485 applications benefit from differential signaling over twisted-pair cable, because noise from external sources couple equally into both signal lines as common-mode noise, which is rejected by the differential receiver input.

Industrial RS-485 cables are of the sheathed, unshielded, twisted-pair type, (UTP), with a characteristic impedance of 120 W and 22–24 AWG. Figure 5-1 shows the cross-section of a four-pair, UTP cable typically used for two full-duplex networks. Similar cables, in two-pair and single-pair versions, are available to accommodate the low-cost design of half-duplex systems.

GUID-517E0D7D-4E4F-4C07-BFD1-5D8415284168-low.gif Figure 5-1 Example of RS-485 Communication Cable

Beyond the network cabling, it is mandatory that the layout of printed-circuit boards and the connector pin assignments of RS-485 equipment maintain the electrical characteristics of the network by keeping both signal lines close and equidistant to another.