A BACnet Analog Input object carries its own Object_Name, Units, and Present_Value — so a client can read what a point is and means without a separate vendor document.

True. Self-description is the defining BACnet idea. Each object exposes named, typed properties — name, units, description, present value, limits — so a client can walk into a building it has never seen, query a controller, and build a working point list. That's the opposite of Modbus, where the vendor's manual is the only thing that knows what the bits mean.

Which identifier must be globally unique across the entire BACnet internetwork?

The device instance number (e.g. device:1001). The device instance number (0 to 4,194,302) uniquely identifies a controller across the whole network. An object instance number only has to be unique within its device — AI:1 can exist on every controller. Two devices that both claim device:1001 is a classic, hard-to-spot integration fault.

A thermostat exposes a mode point with the states AUTO / HEAT / COOL / OFF. Which BACnet object family fits?

Multi-state (MSI / MSO / MSV). Three or more named states means multi-state. The state names live in the object's State_Text property and are addressed by a 1-based integer (1 = AUTO, 2 = HEAT, …). Binary objects hold exactly two states (ACTIVE / INACTIVE); analog objects hold a 32-bit float.

You're polling a controller for one graphic's worth of points — thirty properties across several objects. Which service does this in a single request?

ReadPropertyMultiple. ReadPropertyMultiple (and its write twin WritePropertyMultiple) fetch many properties in one request — the network overhead is the same for one property or thirty. A real graphic poll uses RPM, not a thousand individual ReadProperty calls, which is why a chatty integration that does one-at-a-time reads loads a network far harder than it needs to.

SubscribeCOV lets a device notify the client whenever a value changes by more than its COV_Increment — push instead of poll.

True. Change-of-Value flips the polling model: the client subscribes once and the device sends a notification each time the value moves enough to matter. Where Modbus forces a client to poll fast enough to catch a change, BACnet lets the device announce it. Subscriptions have a lifetime, so the client re-subscribes before it expires.

On a commandable AO, slot 8 (manual override) holds 0 %, slot 16 (BMS sequence) holds 65 %, and every other slot is null. What is the object's Present_Value?

0 % — the lowest-numbered non-null slot wins.. Present_Value resolves to the value in the lowest-numbered non-null slot. Slot 8 sits below slot 16, so the manual override wins and the output reads 0 %. Slot 1 is highest priority, slot 16 lowest; if every slot were null the object would fall back to Relinquish_Default.

A damper is pinned by a months-old hand override at slot 8 while the sequence keeps writing slot 16. How do you hand control back to the sequence?

Write null to slot 8.. Writing null to slot 8 releases it — the slot goes empty, the lowest non-null slot becomes slot 16, and the sequence takes over with nothing else changed. Writing a value (even the right one) into slot 8 just leaves the override in place at a different number. Forgetting to release is the most common way priority-array logic looks "broken": the graphic shows the resolved value, not what the sequence is writing.

An integration maps a three-state fan command into the BMS like this. What breaks? source object: MSV:4 Fan_Mode (1=OFF, 2=LOW, 3=HIGH) mapped as: BV:21 Fan_Cmd (INACTIVE / ACTIVE)

A Binary Value only holds two states, so LOW and HIGH collapse into one — the third state is unreachable.. A Binary object is strictly two-state (ACTIVE / INACTIVE). A three-state multi-state command can't round-trip through it — you lose the distinction between LOW and HIGH, and the BMS can never command the third state. The fix is to map it to a Multi-state Value and carry the State_Text, or expose the states some other faithful way. Matching the object family to the data is part of reading a point list correctly.

What UDP port does BACnet/IP use by default? (It's 0xBAC0 in hex — the source of the "BACnet" port mnemonic.)

47808. The IANA-registered BACnet/IP port is UDP 47808, which is 0xBAC0 in hex. Knowing the hex form is what lets you recognize a device address that ends in BAC0 — and spot one ending in BAC1 (47809) as a device on a second, non-default BACnet/IP network.

Move a controller from an MS/TP bus to BACnet/IP and its object model, property names, services, and priority array all stay identical — only the framing around each message changes.

True. MS/TP and BACnet/IP are two data-link options for the same protocol logic. The objects, properties, services, and priority array are defined once in ASHRAE 135 and don't care what's underneath; only the wrapper differs — RS-485 token-passing frames versus UDP datagrams on port 47808. What does change across the boundary is the broadcast story, which is BACnet Networking's subject.

Practice · BACnet Basics

BACnet Basics Quiz BACnet

Read the BACnet Basics lesson first, then test yourself — ten questions on the fundamentals: the self-describing device / object / property model, the analog / binary / multi-state families, the everyday services, and the priority array's lowest-non-null-slot rule (plus how writing null releases an override). Each reveal links back to the section of the lesson it came from.

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