BACnet MS/TP devices randomly going offline

Demo Engineer · Controls Engineer2026-08-11

BACnetMS/TPTroubleshooting

We have a BACnet MS/TP trunk with about 24 field controllers on it. Every few days, a handful of them drop offline in the BMS front end for 10–15 minutes and then come back on their own. No pattern by device address that I can see. Trunk was tested at commissioning with no wiring faults. Looking for a systematic checklist before I start swapping hardware.

Manufacturer

Tridium

Protocol

BACnet MS/TP

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3 Answers

Demo Engineer · Senior Controls Engineer

Accepted

Start with trunk loading before touching devices. Count total MS/TP devices against the segment limit for your baud rate, check end-of-line termination at both physical ends only, and verify bias resistors aren't duplicated. Intermittent, self-recovering dropouts across random addresses usually point to token-passing timeouts from an overloaded trunk rather than a single bad device.

14 upvotes · 2026-08-12

Demo Engineer · BMS Commissioning Specialist

Also worth logging baud rate mismatches — one controller defaulting to auto-baud after a power blip can disrupt the whole segment until it resyncs. Pull the network statistics from the supervisor and look for token rotation time creeping up before a dropout event.

9 upvotes · 2026-08-12

Demo Engineer · Field Technician

In our case it turned out to be a shared conduit run picking up EMI from a nearby VFD panel. Worth checking cable routing near drive panels if the electrical noise theory hasn't been ruled out yet.

5 upvotes · 2026-08-13