BACnet MS/TP — token-passing RS-485
BACnet MS/TP (Master-Slave / Token-Passing) runs the BACnet application protocol over RS-485. Only the node holding the token may transmit. Distinctive trait: watch the amber token chip pass between nodes before any data frame is allowed onto the bus. Default 76,800 baud — slow enough that you can see the token rotation cycle.
Animation
Engineering pitfalls
Mixed baud rates on the same segment
BACnet MS/TP requires every node on a segment to use the same baud. One mis-configured node = the whole segment goes silent. Always document baud as part of commissioning.
Token-rotation timeout under-provisioned
If a node drops out, the master waits for token timeout before initiating recovery. Set too short = false drops; too long = service interruption while bus reconfigures. Tune per node count.
Max-master setting incorrect
Each node has a Max_Master property that defines the highest MAC address it polls for the token. Set lower than your actual highest MAC and that node is invisible. Audit against the device-list during install.
Reply-too-late frames cause re-poll storms
If a slave replies after the token has rotated away, the master treats the reply as garbage and re-polls. Storm = effective bus collapse. Check reply timing against the baud-derived character time budget.
References
Primary sources
- ASHRAE 135 — BACnet — A Data Communication Protocol for Building Automation and Control Networks.
- ASHRAE 135 Annex H — MS/TP Master-Slave/Token-Passing.
- TIA-485-A — Electrical characteristics of RS-485 differential transmitters.