A BACnet MS/TP network may have three different numbers that look like addresses: the local MAC address, the Device Object instance number, and Max Master. Confusing them can produce duplicate identities or prevent a device from joining the bus. Each setting has a separate purpose.
Three numbers with three different jobs
The MS/TP MAC identifies a device on its local RS-485 data-link network. The Device Object instance identifies a BACnet device across the routed BACnet internetwork. Max Master controls the highest master MAC address considered by the token-passing search process.
Neither Device ID nor Max Master is a substitute for the device’s actual MS/TP MAC. Typing a six-digit Device ID into a seven-position binary address bank cannot reproduce that identifier.
Vendor documentation from Siemens distinguishes local MS/TP MAC addresses from the globally unique BACnet Device Object identifier. This is why a commissioning worksheet needs separate columns for both.
MS/TP MAC address belongs to one trunk
MS/TP masters use local MAC addresses from 0 through 127 under the BACnet standard, but individual controllers can reserve some values. A local MAC must be unique among devices on that MS/TP network.
Imagine a building with a first-floor MS/TP trunk and a second-floor trunk connected through routers. A controller with MAC address 5 on the first trunk does not collide merely because a separate second-floor trunk also uses MAC 5.
The controller manual determines whether MAC 5 is selected using a DIP switch, touchscreen, jumpers, or software configuration. Use our BACnet MS/TP DIP switch address calculator only when the device’s supported mapping is documented.
Device Object instance spans the internetwork
BACnet devices have an object identifier with a Device Object instance component. This identifier must be unique across the BACnet internetwork so discovery and object references can resolve the intended equipment.
For example, a controller might have local MS/TP MAC 5 and Device Instance 120005. Another trunk can reuse MAC 5, but the device instance must not duplicate 120005 within the same BACnet internetwork.
Many integrators use building numbers, floors or panel codes as part of a Device ID assignment plan. Those conventions help people recognize devices, but BACnet does not require the Device ID to equal the MAC.
Max Master is a search limit, not a device count
BACnet MS/TP master nodes participate in token passing. The Max Master property limits the highest master MAC searched during discovery of other masters on a trunk.
If a trunk contains masters at MACs 3, 8 and 14, Max Master must not be set below 14 on equipment that needs to discover those devices. A setting of 10 might keep the device at 14 from joining the expected token-passing cycle.
A very high search limit can also waste time checking unused MAC values. Follow the device’s Max Master configuration rules and allow room for planned future additions where necessary. Some devices manage the setting automatically.
Commissioning checklist with examples
Before making changes, record the controller model, trunk name, assigned local MAC, Device ID, serial baud setting, and Max Master. Make sure your device list includes routers and third-party controllers where relevant.
Verify unique supported MACs on each trunk, unique Device IDs throughout the routed internetwork, and Max Master values that cover the actual highest master address. Then review RS-485 polarity, termination and baud consistency.
A simple worksheet can have columns for network name, MAC, Device ID, Max Master, baud rate, status and comments. Keep revisions so technicians can tell whether a missing device was never assigned or stopped working after maintenance.
How to diagnose a missing controller
A controller that fails discovery is not automatically suffering from an address collision. Check power, wiring, correct serial settings and whether the router exposes the right BACnet network first.
If two controllers share the same local MAC, use our duplicate BACnet MAC troubleshooting guide. If their MACs are distinct, check Device ID duplication and whether Max Master reaches the highest master address.
For physical bus problems, consult RS-485 termination switch guidance. Changing random DIP switches is less useful than isolating one confirmed fault at a time.
Device-specific DIP switch caveats
Some controllers reserve switch positions for communication mode, a software-address override, or other features. Do not assume all eight positions in a switch bank encode address bits.
Different devices may require a restart before new MAC switch settings become active. Record original values and follow the commissioning instructions so the change can be reversed if the controller disappears.
Live HVAC and fire-related systems may have additional operational requirements. Schedule address changes through the site owner and qualified building automation personnel.
Example commissioning worksheet
Consider two BACnet trunks joined through appropriate routers. Trunk A has controllers at MAC 3 and MAC 8, while Trunk B has a controller at MAC 3. Reusing MAC 3 on a separate local MS/TP network is not the same as an address collision on one bus.
Give the controllers distinct Device Object instance numbers across the BACnet internetwork, such as 100003, 100008 and 200003. The exact numbering plan is a site convention. The principle is that Device IDs identify different BACnet devices and do not need to match local MACs.
If another controller at MAC 26 joins Trunk A, review the Max Master values on relevant masters. A setting below the new highest address can interfere with token discovery. Max Master is the highest searched master MAC, not the number of installed controllers.
Before changing address switches
Document the original position of every DIP switch and any software address override. Some devices require a restart for new switch settings to take effect, while others reserve one switch for an unrelated mode.
Make the change only after confirming the exact controller manual and affected network. Then verify the local MAC, Device ID, Max Master and trunk health independently instead of treating one successful communication test as complete commissioning.
Frequently Asked Questions
Are BACnet MAC and Device Instance the same?
No. MAC is local to an MS/TP network while Device Instance is an internetwork-wide identity.
Is Max Master the total number of devices?
No. It is the highest MS/TP master MAC address to include in the search range.
Can different trunks use the same MAC number?
Yes. On independent BACnet MS/TP networks, the same local MAC number can be used without a local collision.
Do all BACnet devices use DIP switches?
No. Configuration can use hardware switches, a service interface or software settings, depending on the equipment.
Official resources
Siemens BACnet architecture and addressing documentation
ABB BACnet datalink configuration reference
Technical note: Use the exact equipment manual before changing operational settings. This article is educational and does not replace qualified technical service.