Best to Worst BACnet MSTP Topology (RS485)
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When designing a BACnet MS/TP network over RS-485, topology matters more than many engineers initially realize. Even when all devices support BACnet MS/TP correctly, poor wiring practices can lead to intermittent communication issues, token passing failures, excessive retries, or complete network outages.
In this article, we rank BACnet MS/TP (RS-485) wiring topologies from best to worst based on reliability, signal integrity, and ease of troubleshooting. “Best” here means the topology that is least likely to cause communication problems over time.
Best Topology Arrangement (Recommended)
The best BACnet MS/TP topology is a single continuous trunk that enters and exits each device directly through its RS-485 terminal block. This is often referred to as a “daisy-chain” or “in-out” configuration.
This approach minimizes signal reflections and impedance mismatches, allowing the differential RS-485 signals to propagate cleanly along the bus. Proper termination resistors should be installed only at the two physical ends of the trunk.
This topology is the easiest to commission, the most tolerant of cable length variations, and the least likely to introduce intermittent communication faults.
Getting Worse: Drop or Stub Connections
As networks move away from direct in-out connections and begin using drops (stubs), signal quality degrades. These drop cables create additional electrical paths where reflections and harmonics can form.
Long stubs are particularly problematic. If the drop cables are not twisted, or if they run near sources of electrical noise, interference can be induced onto the bus. This often results in sporadic failures that are difficult to reproduce or diagnose.
While short drops may function in small systems, they should be avoided whenever possible in production installations.
Worst Topology: Star Configuration
Star topologies should be avoided entirely for BACnet MS/TP over RS-485. In a star configuration, multiple cable branches radiate from a central point, creating severe impedance mismatches.
These reflections can distort the differential signal enough to break token passing, cause missed frames, or lead to devices randomly dropping off the network.
Star networks are also significantly harder to troubleshoot. Problems may appear or disappear depending on cable length, device power state, or environmental noise, making long-term reliability poor.
Key Takeaways for Reliable BACnet MS/TP Networks
- Use a single continuous RS-485 trunk whenever possible
- Avoid long stubs or drop connections
- Never use star topologies for BACnet MS/TP
- Install termination resistors only at the ends of the trunk
- Use twisted pair cable and proper grounding practices
Following these guidelines will dramatically improve the reliability and stability of your BACnet MS/TP network and reduce commissioning and maintenance costs over the life of the system.