What You-Are Is
You-Are is a remote-device-management service in BACnet, defined in Clause 16.11 of ANSI/ASHRAE 135-2024 and added at Protocol_Revision 24. A commissioning tool uses it to configure a remote device’s identity: its Device object instance number, its MAC address, or both. It is an unconfirmed service. You-Are is the tool’s half of the zero-touch commissioning pair; the device asks for an identity with Who-Am-I, and the tool answers with You-Are.
It is how a device gets its network-wide identity assigned remotely instead of by hand. The device is identified by the same three values Who-Am-I carries: vendor identifier, model name, and serial number.
How It Works
The tool sends You-Are carrying the vendor identifier, model name, and serial number of the device it means to configure, plus the identity to assign: a Device object identifier, a device MAC address, or both. A device accepts the request only if all three identifying values match its own Device object; the message is unicast when the tool knows the device’s MAC address and broadcast when it does not. The matching device changes its Device instance number and, where changeable, its MAC address, and then broadcasts an I-Am so the rest of the network learns its new identity. It keeps the assigned values across restarts. Assigning the instance number 4194303 does the reverse: it returns the device to the unconfigured state, ready to announce with Who-Am-I again.
Where You’ll See It
- Zero-touch commissioning. A tool assigns each new device its instance number and address from serial-number-matched You-Are requests, with no laptop on the device.
- Renumbering. Correcting or reassigning a device’s instance number over the network during a site reorganization.
- Reset to unconfigured. Sending You-Are with instance 4194303 to return a device to the unconfigured state before re-provisioning it.
You-Are Error Codes
You-Are is an unconfirmed service, so it has no acknowledgment and returns no application-layer error. Its success signal is indirect: a configured device broadcasts an I-Am with its new identity, and the absence of that I-Am is how a tool infers the assignment did not take. The usual causes:
- The vendor identifier, model name, or serial number in the You-Are did not exactly match the target device, so no device accepted it.
- The device is an MS/TP (Master-Slave/Token-Passing, BACnet’s RS-485 serial transport) subordinate node, which does not send the confirming I-Am.
- The You-Are was broadcast but did not cross to the device’s IP subnet because there is no BACnet BBMD path.
Profiles That Require It
You-Are is a Protocol_Revision 24 zero-touch commissioning feature, not part of the required BIBB set for the established device profiles. Sending You-Are is a commissioning-tool and operator-workstation behavior on the device-address BIBB family (DM-DDA), appearing in advanced workstation profiles such as B-AWS; the device side that accepts it pairs with Who-Am-I. Support is declared in a device’s PICS (Protocol Implementation Conformance Statement); check Protocol_Revision in the Device object to know whether a device is new enough to use it.
What Chipkin Provides
The CAS BACnet Stack supports You-Are in both directions, as a tool assigning identities and as a device accepting one. Talk to us about adding BACnet client or server support to your device.