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

3 Network Topology

The RS-485 standards suggests that its nodes be networked in a daisy-chain, also known as party line or bus topology (see Figure 3-1. In this topology, the participating drivers, receivers, and transceivers connect to a main cable trunk via short network stubs. The interface bus can be designed for full-duplex or half-duplex transmission (see Figure 3-2).

GUID-DEA88D34-B1D6-4FC5-A544-C5C36A9F8A26-low.gif Figure 3-1 RS-485 Bus Structure

The full-duplex implementation requires two signal pairs, (four wires), and full-duplex transceivers with separate bus access lines for transmitter and receiver. Full-duplex allows a node to simultaneously transmit data on one pair while receiving data on the other pair.

GUID-0E534496-F8B8-45E7-BD5D-C5A82FE13A90-low.gif Figure 3-2 Full-Duplex and Half-Duplex Bus Structures in RS-485

In half-duplex, only one signal pair is used, requiring the driving and receiving of data to occur at different times. Both implementations necessitate the controlled operation of all nodes via direction control signals, such as Driver/Receiver Enable signals, to ensure that only one driver is active on the bus at any time. Having more than one driver accessing the bus at the same time leads to bus contention, which, at all times, must be avoided through software control.