BACnet MS/TP
BACnet MS/TP runs over a shared serial bus. Check bus timing, addresses and topology alongside the objects and services used by the application.
Sources and scopeSource record 25 August 2026
Technical source record: 25 August 2026. Check the linked documentation for current product requirements.
Normative state machines and electrical constraints require the licensed BACnet standard and current product manuals.
On this page
Overview#
BACnet Master Slave/Token Passing (MS/TP) is a BACnet data link normally carried on an EIA 485 multidrop bus. Master nodes pass a token to control transmission; slave only nodes respond but don't hold the token. BACnet routers commonly bridge an MS/TP segment into BACnet/IP or BACnet/SC.1
Three address spaces#
Keep these separate in configuration and logs:
- the MS/TP MAC address on one serial segment;
- the BACnet network number assigned to that segment;
- the BACnet device instance at the application layer.
Duplicate MAC addresses corrupt token operation. Duplicate device instances create application ambiguity even when MACs differ. A router port must not reuse a network number already present elsewhere in the internetwork.
Timing and token health#
Correctness depends on baud, data format, bus wiring, turnaround, token timers, maximum master address, maximum frames per token, and implementation state machine behaviour. Don't copy a generic serial profile without checking every node. A high Max_Master expands token search time; an incorrect low value makes higher address masters unreachable.
Monitor token rotation time, retries, frame/CRC errors, silence, duplicate tokens, unexpected masters, router queueing, and device restart patterns. Rate limit upstream polling so the router can't saturate the serial segment. COV subscription traffic still consumes MS/TP bandwidth.
Installation and diagnostics#
Record topology, cable, segment length, baud, termination, bias, reference conductor/ground practice, isolation, MAC allocation, network number, router port, and power domain boundaries. Use serial transports for the distinction between protocol and electrical layer.
Passive observation should use an isolated, high impedance interface. Adding a poorly configured USB adapter can change bias/termination or transmit unexpectedly. Active token or discovery tools require an isolated bench or explicit site authorisation.
Security boundary#
MS/TP has no native per frame confidentiality or cryptographic source authentication. Physical access, cabinet security, isolated routing, strict router ACLs, and application level write controls are therefore material. BACnet/SC can secure the routed IP backbone but doesn't encrypt or authenticate an attached legacy MS/TP segment; the router is a security boundary.