What It Is
Token passing is the media-access method used by BACnet MS/TP. Only the master device currently holding the token can initiate communication on the RS-485 trunk.
That makes BACnet MS/TP behavior much more sensitive to trunk design than BACnet/IP. A wiring, MAC, or serial-parameter issue often shows up as an application problem only because token rotation is being disrupted underneath it.
How It Works
- One master holds the token.
- It sends traffic up to its allowed work window.
- It passes the token to the next master MAC address.
- If the expected next master does not respond, the network searches onward up to Max Masters.
Slave devices do not hold the token. They answer only when polled by a master.
Recovery Behavior
If the token is lost, the remaining masters initiate Poll For Master behavior to re-establish token rotation. Short interruptions are normal during recovery. Frequent recovery events are not.
| Symptom | Likely Underlying Issue |
|---|---|
| Repeated short communication gaps | Intermittent device or wiring issue |
| Random device disappearances | Duplicate MAC or unstable master behavior |
| Slow overall polling | Too many masters or inefficient Max Masters setting |
[!NOTE] Frequent token-loss recovery is usually a wiring, addressing, or serial-discipline problem, not a mysterious BACnet object-model problem.