BACnet MSTP auto addressing

Every device on a MSTP bus should have a unique MAC address.  For master device, the available address range is 0~127,  and 128~254 for slave device.

Usually MAC address is set by DIP switch, jumper, LCD screen, firmware downloaded by configuration tools. Some devices support MAC address modification through BACnet object/property, but before doing that, it should have a valid MAC address to join BACnet network.

If the unique MAC address could be automatic obtained like we get IP address just by plugging notebook into home/office network, it would save a lot of time in commission.

There are several solutions had been discussed.    Now seems committee prefer  “Zero-Config” (addendum 135-2012bb)

BACnet stack has implemented “Zero-Config”.

“Zero-Config” only works on fixed configuration that Max-master is 127 and automatic assigned address range is 64~127. If not, it may cause mess.

To avoid above limitation, BACRouter implements proprietary auto addressing solution and keep compatible with “Zero-Config”.  It has some attractive features:

  1. Learning Max-master from bus traffic.
  2. Assigning MAC address from highest unused one.

So users have more freedom on MAC address schema,  For example, leave address 0~30 for fixed address devices, set Max-master as 40, so automatic addressing devices would use 31~40.

Both Zero-Config and BACRouter’s solution have trouble when a automatic addressing device is pulled out bus then plugged in again without reboot, because a new attached automatic addressing device would occupy the same address.(BACRouter is more weak in such situation because of it’s predictable address assigning), So

ALWAYS power on automatic addressing device after attaching to bus.