What It Is
BACnet MS/TP is the serial BACnet transport that runs over RS-485. It uses token passing between master devices, which makes bus quality, addressing discipline, and Max Masters settings central to stable communication.
How Token Passing Works
Only the device holding the token can initiate traffic. That makes MS/TP behavior very different from BACnet/IP, where network access is not controlled by a rotating token.
If one device has the wrong MAC, wrong baud rate, or a broken serial path, the visible symptom often looks like a random BACnet problem even though the failure is really at the trunk-design level.
Key Parameters
| Parameter | Typical Range | Why It Matters |
|---|---|---|
| Baud rate | 9600 to 115200 | Every device on the trunk must match |
| MAC address | Masters 0 to 127 | Must be unique per trunk |
| Max Masters | 1 to 127 | Must be at or above the highest master MAC |
| Max Info Frames | Device-specific | Affects how much work one device does per token |
Common Failure Modes
| Symptom | Likely Cause | First Check |
|---|---|---|
| Some devices never appear | Max Masters is too low | Highest master MAC on the trunk |
| Intermittent communication | Duplicate MAC or poor RS-485 discipline | MAC plan, shield, termination, polarity |
| High CRC or framing errors | Serial mismatch or noise | Baud rate, grounding, cable path |
| One tool sees the network but the BMS behaves poorly | BMS assumptions differ from trunk reality | Compare production polling path to analyzer evidence |
Tools & Diagnostics
| Tool | Type | Description |
|---|---|---|
| CAS BACnet Explorer | Explorer | Object-level validation once the trunk is healthy |
| Wireshark | Packet analyzer | Useful where an MS/TP capture path exists or when validating upstream BACnet/IP exposure |
| QuickServer | Gateway and diagnostics platform | Useful when BACnet MS/TP points must be exposed as Modbus or other protocols while preserving a clean supervisory handoff |