BACnet MS/TP: Duplicate Device ID Conflict and Resolution

This article explains what occurs when two BACnet MS/TP devices are configured with the same Device ID on a single MS/TP network. It describes protocol behavior, observable symptoms, and why duplicate Device IDs violate the BACnet standard even when MAC addresses differ.

This topic focuses on Device ID conflicts. For guidance on MS/TP network sizing, baud rates, and bandwidth considerations, see: BACnet MS/TP network capacity and device limits .

BACnet Device ID and MS/TP Addressing

In BACnet, the Device ID is a globally unique identifier used at the application layer to distinguish BACnet devices. The BACnet standard explicitly requires that no two devices on the same BACnet internetwork share the same Device ID.

On MS/TP networks, each device also has a MAC address, which determines token-passing order at the data link layer. While MAC addresses must be unique on a segment, they do not replace the requirement for unique Device IDs.

MS/TP Token-Passing Behavior

BACnet MS/TP uses a deterministic token-passing mechanism. Only the device holding the token may transmit messages. After completing transmissions, the device passes the token to the next device with a higher MAC address. When the highest MAC address is reached, the token wraps to the lowest.

Example MS/TP segment:

Device A: MAC address 0x03
Device B: MAC address 0x04
Device C: MAC address 0x05

Token flow follows the MAC address order: A → B → C → A.

Duplicate MAC Address vs Duplicate Device ID

Duplicate MAC addresses on an MS/TP segment cause immediate network instability. Both devices believe they own the token and will attempt to transmit simultaneously, resulting in collisions, token loss, and unpredictable behavior.

Duplicate Device IDs with different MAC addresses behave differently. While still illegal per the BACnet standard, the resulting behavior is deterministic due to token passing.

Duplicate Device ID Example

Consider the following configuration:

Device A: MAC 0x03, Device ID 7
Device B: MAC 0x04, Device ID 7
Device C: MAC 0x05, Device ID 13

When Device C issues a broadcast Who-Is or property request targeting Device ID 7:

All devices receive the broadcast request.
Devices A and B both prepare responses.
The token is passed to Device A (lower MAC address).
Device A responds and passes the token to Device B.
Device B responds and passes the token back to Device C.
Device C receives the second response and overwrites the first.

This behavior aligns with BACnet address binding rules, where the most recent I-Am or response replaces previous bindings for the same Device ID.

Engineering Implications

Although duplicate Device IDs with unique MAC addresses produce predictable results, they violate the BACnet specification and can lead to subtle and difficult-to-diagnose issues, including:

Inconsistent data values
Unexpected device bindings
Intermittent overwriting of property values
Commissioning confusion and troubleshooting delays

For these reasons, all BACnet devices on an internetwork must be configured with unique Device IDs.

Conclusion

Duplicate MAC addresses on an MS/TP network cause immediate instability and must be avoided. Duplicate Device IDs, even with different MAC addresses, violate the BACnet standard and lead to deterministic but incorrect behavior.

Proper Device ID management is essential for stable BACnet MS/TP operation and reliable system integration.

If you have questions related to BACnet MS/TP configuration, protocol behavior, or integration troubleshooting, contact Chipkin Automation Systems.