Easy BACnet
BACnet glossary: what do all the terms mean?
Updated 11 October 2026
Short answer
Plain-English definitions of the BACnet terms you'll run into on the job. A device is one controller; an object (usually called a point) is one value inside it; Present_Value is its current reading or command. Device ID, object types, the priority array, Who-Is/I-Am, BBMDs and MS/TP are below, each linked to a longer guide.
The basics
| Term | What it means |
|---|---|
| BACnet | The open building-automation protocol defined by ASHRAE 135 / ISO 16484-5. See What is BACnet? |
| Device | One controller, gateway or piece of equipment on the network. Has a Device ID and, on BACnet IP, an IP address. |
| Object | One thing inside a device: an input, an output, a setpoint, a schedule. Has a type and an instance number. |
| Point | The everyday word for an object you read or command. A device's full set is its points list. |
| Property | A field on an object, e.g. Object_Name, Present_Value, Units, Status_Flags. |
| Present_Value | The current reading (inputs) or current command/value (outputs and values). |
| Object_List | A property of the Device object listing every object in it. Reading it is how a tool builds the points list. |
Identifying and finding things
| Term | What it means |
|---|---|
| Device ID | The device's instance number, 0–4194302, unique across the whole BACnet network (including routed MS/TP). Not the IP address. 4194303 is reserved as a wildcard. |
| Object type | What kind of object it is: Analog Input (AI), Binary Output (BO), Multi-state Value (MSV) and so on. |
| Instance number | Which one of that type in the device: AI 1, AI 2, AV 3001. |
| Object Identifier | Type and instance packed into one 32-bit number (10 bits type, 22 bits instance). Our decoder tool splits it apart. |
| Who-Is / I-Am | The broadcast request and the replies that let a tool discover devices without knowing their addresses. |
| Segmentation | Splitting a large reply across several packets. Small controllers often don't support it, which is why tools read a big Object_List one entry at a time. |
Commanding and overrides
| Term | What it means |
|---|---|
| Priority array | 16 slots on a commandable object. The lowest-numbered slot holding a value wins (1 is highest). |
| Command priority | The slot (1–16) you write at. 8 is conventionally Manual Operator; 1–2 are life safety, 5 critical equipment, 6 minimum on/off; programs often use 10–16. |
| Relinquish_Default | The value the object uses when every priority slot is empty. |
| Release (relinquish) | Writing NULL to a slot to clear it. See how to release an override. |
| Commandable | Has a priority array. Analog/Binary/Multi-state Outputs always do; Values (AV, BV, MSV) only if the vendor built them that way. Inputs never do. |
| Out_Of_Service | A property that disconnects Present_Value from the physical I/O so it can be written for testing. |
| Status_Flags | Four flags on a point: IN_ALARM, FAULT, OVERRIDDEN, OUT_OF_SERVICE. |
Networking and wiring
| Term | What it means |
|---|---|
| BACnet IP | BACnet in UDP over Ethernet, normally on port 47808 (0xBAC0). What a phone can reach. |
| MS/TP | BACnet over an RS-485 twisted pair, common for VAVs and unit controllers. Devices are addressed by a MAC (0–127 for masters). A phone can't reach it directly. |
| BACnet router | Connects BACnet networks, typically MS/TP trunks to BACnet IP, so routed devices can be reached from IP. |
| Network number | The BACnet number of a network segment (e.g. each MS/TP trunk). Routed devices are addressed by network number plus MAC. |
| BBMD | BACnet/IP Broadcast Management Device. Forwards BACnet broadcasts such as Who-Is between IP subnets, and accepts foreign-device registrations. |
| Foreign device | A tool on another subnet that registers with a BBMD so it can send and receive broadcasts through it. |
| Subnet | The IP range your phone is on. A broadcast scan only reaches devices on the same subnet unless a BBMD is used. |
| BACnet/SC | Secure Connect, a newer TLS-based BACnet transport. Easy BACnet doesn't speak it; it's BACnet IP only. |
New to this? Start with What is BACnet?, then come back here when a term trips you up.
Check it on your own network. Easy BACnet is a free Android app that finds BACnet IP devices and reads their points in a couple of taps.
Get it on Google Play