Mixed BACnet Cable Gauges Causing Device Dropouts

Problem

BACnet MS/TP daisy chain works fine when tested individually but devices in the middle of the chain drop offline when all units power up simultaneously. Problem appears intermittent or load-dependent.

Root Cause

Mixed cable gauges (e.g., 18 AWG and 20 AWG on same daisy chain) cause impedance mismatches, signal reflections, and collisions on the bus. While each gauge meets the spec individually, mixing them violates MS/TP wiring standards.

Solution

1. Verify the actual wiring installed:

   • Physically inspect the daisy chain and note cable gauge on each segment
   • Check the "as-built" drawings against the design spec

2. If gauges are mixed:

   • Replace all segments with uniform gauge (preferably 20 AWG or better, per EIA-485 spec)
   • Use 2-core shielded cable rated for RS-485 (not generic low-voltage wire)
   • Ensure all terminators are 120Ω terminating resistors at both ends of the daisy chain

3. Isolate problem segments (before replacing cable):

   • Power off one segment and verify communication on the rest
   • Gradually power segments back on to identify which one(s) cause dropouts
   • Focus replacement on identified problem segments first

4. After replacing cable:

   • Verify all baud rates match (typically 9600, 19200, or 38400 bps)
   • Check all MAC addresses are unique
   • Power-cycle all devices on the chain to refresh discovery

5. Monitor for improvements:

   • Log communication errors for 24–48 hours to confirm stability
   • Adjust MS/TP parameters (Max Masters, Max Info Frames) if dropouts persist