What is an MS/TP MAC address?
By Bassirou Thiam, building automation engineer · Updated
The MS/TP MAC address is a device's address on its RS-485 trunk: 0 to 127 for master devices, up to 254 for slaves. It must be unique on that trunk only. Many specs require sequential addresses starting at 0, with Max Master set to the highest master address.
Why sequential addresses
Masters pass the token by polling addresses up to Max Master. Gaps and a Max Master set too high make the token search for devices that do not exist, which slows the trunk.
Sources
- KMC Controls, Application Note AN0404A – Planning BACnet networks
- Siemens, guide specification Section 23 09 00 – Building Automation System (submittals, network, point naming)
- AutomatedBuildings.com, BACnet numbers for MS/TP MAC addresses, device instances and BACnet networks (Contemporary Controls, 2025)
Related
GlossaryWhat is BACnet MS/TP?Definition of BACnet MS/TP (master-slave/token-passing) over RS-485: how it works, addressing, and the segment limits that shape a BAS riser.
GlossaryWhat is a BACnet device instance?Definition of the BACnet device instance: the number that identifies a device across the whole internetwork, valid range and uniqueness rule.
GuidesBACnet device instance numbering and MS/TP addressingThe rules for BACnet device instances, MS/TP MAC addresses, Max Master and network numbers, with segment limits and a numbering scheme you can reuse.