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

11 Grounding and Isolation

When designing a remote data link, the designer must assume that large ground potential differences (GPD) exist. These voltages add as common-mode noise, Vn, to the transmitter output. Even if the total superimposed signal is within the receiver’s input common-mode range, relying on the local earth ground as a reliable path for the return current is dangerous (see Figure 11-1a).

GUID-20DEECEF-6AB0-4F8D-8E50-B99A55A1364A-low.gif Figure 11-1 Design Pitfalls to be Aware off: a) High GPD, b) High Loop Current, c) Reduced Loop Current, Yet Highly Sensitive to Induced Noise Due to Large Ground Loop

Because remote nodes are likely to draw their power from different sections of the electrical installation, modification to the installation, (i.e., during maintenance work), can increase the GPD to the extent that the receiver’s input common-mode range is exceeded. Thus, a data link working today might cease operation sometime in the future.

The direct connection of remote grounds through ground wire also is not recommended (see Figure 11-1b), as this causes large ground loop currents to couple into the data lines as common-mode noise.

To allow for a direct connection of remote grounds, the RS485 standard recommends the separation of device ground and local system ground via the insertion of resistors (Figure 11-1c). Although this approach reduces loop current, the existence of a large ground loop keeps the data link sensitive to noise generated somewhere else along the loop. Thus, a robust data link has not been established yet.

The approach to tolerate GPDs up to several kilovolts across a robust RS-485 data link and over long distance is the galvanic isolation of the signal and supply lines of a bus transceiver from its local signal and supply sources (see Figure 11-2).

GUID-234FA4C6-6FCA-41F1-A3B7-C46F5FF5CEA3-low.gif Figure 11-2 Isolation of Two Remote Transceiver Stations With Single-Ground Reference

In this case, supply isolators, such as isolated DC/DC converters, and signal isolators, such as digital, capacitive isolators, prevent current flow between remote system grounds and avoid the creation of current loops.

Whereas Figure 11-2 shows the detailed connection of only two transceiver nodes, Figure 11-3 gives an example for multiple, isolated transceivers. All transceivers but one connect to the bus via isolation. The non-isolated transceiver on the left provides the single-ground reference for the entire bus.

GUID-F14C76ED-8AB2-4CBB-B4A7-E569BDE69630-low.gif Figure 11-3 Isolation of Multiple Fieldbus Transceiver Stations