What It Is
I-Am is the BACnet service a device uses to identify itself after a Who-Is request or when it announces itself on the network. It tells clients which BACnet device responded and enough detail to begin normal communication.
What An I-Am Contains
| Field | Description |
|---|---|
| Device Object Identifier | Includes the responding Device Instance |
| Max APDU Length Accepted | Shows the largest application payload the device can handle |
| Segmentation Supported | Indicates whether segmented messaging is available |
| Vendor ID | Identifies the vendor assignment associated with the device |
Broadcast Versus Unicast Behavior
Many BACnet devices answer Who-Is with a broadcast I-Am, but the standard also allows unicast responses. That distinction matters because some supervisory platforms and network designs behave differently when responses return only to the original requester.
If one tool sees a device and another does not, checking whether I-Am is broadcast or unicast is often one of the fastest diagnostic steps.
Common Issues
- No
I-Amresponse because the device is offline or outside the discovery path - Unicast
I-Ambehavior that one tool understands and another ignores - Duplicate Device Instance values causing discovery confusion
- Routed BACnet/IP networks where the response never reaches the actual client path