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

9 Data Rate Versus Bus Length

The maximum bus length is limited by the transmission line losses and the signal jitter at a given data rate. Because data reliability sharply decreases for a jitter of 10% or more of the baud period, Figure 9-1 shows the cable length versus data rate characteristic of a conventional RS-485 cable for a 10% signal jitter.

GUID-847C34F1-E729-450D-AC27-551A6975889E-low.gif
Section 1 of the graph presents the area of high data rates over short cable length. Here, the losses of the transmission line can be neglected and the data rate is mainly determined by the driver’s rise time. Although the standard recommends 10 Mbps, today’s fast interface circuits can operate at data rates of up to 40 Mbps.
Section 2 shows the transition from short to long data lines. The losses of the transmission lines have to be taken into account. Thus, with increasing cable length, the data rate must be reduced. A rule of thumb states that the product of the line length [m] times the data rate [bps] should be < 107. This rule is far more conservative than today's cable performance and will therefore show less length at a given data rate than the graph presents.
Section 3 presents the lower frequency range where the line resistance, and not the switching, limits the cable length. Here, the cable resistance approaches the value fo the termination resistor. This voltage divider diminishes the signal by -6 dB. For a 22 AWG cable, 120 W, UTP, this occurs at approximately 1200 m.
Figure 9-1 Cable Length Versus Data Rate