BACnet Device Object

What the Device Object Is

The Device object, defined in Clause 12.11 of ANSI/ASHRAE 135-2024, is the one object every BACnet device has exactly one of. It carries the device’s identity (who it is, who made it, what firmware it runs), its capabilities (which services and object types it supports), its object list, and the network parameters other devices need to talk to it.

The Device object’s instance number is the device’s network-wide identity: the number a client learns from an I-Am and uses to address it. When a tool discovers a device, the first object it reads is the Device object, because everything else about the device is described there.

Where You’ll See It

• Discovery and identification. The device list in a tool like the CAS BACnet Explorer is built from each device’s Device object identity properties.
• Capability checks. Confirming a device supports a service or object type before relying on it means reading Protocol_Services_Supported and Protocol_Object_Types_Supported.
• Commissioning. Setting the device instance, name, and APDU parameters, and reading Protocol_Revision to know which edition of the standard the device implements.

Required Properties

Clause 12.11 requires every Device object to provide these properties. Datatypes are the spec’s names: Unsigned is an integer, CharacterString is text, and the BACnet-prefixed types are structured values defined by the standard.

Property	Datatype	What it holds
Object_Identifier	BACnetObjectIdentifier	The device instance, this device’s network-wide identity
Object_Name	CharacterString	The device name, unique across the internetwork
Object_Type	BACnetObjectType	Always device for this object
System_Status	BACnetDeviceStatus	The device’s overall state (operational, download-in-progress, non-operational)
Vendor_Name	CharacterString	The manufacturer’s name
Vendor_Identifier	Unsigned	The manufacturer’s BACnet vendor ID, assigned by ASHRAE
Model_Name	CharacterString	The product model
Firmware_Revision	CharacterString	The device firmware version
Application_Software_Version	CharacterString	The application or configuration version
Protocol_Version	Unsigned	The BACnet protocol version (1)
Protocol_Revision	Unsigned	The BACnet protocol revision, which edition of the standard the device implements
Protocol_Services_Supported	BACnetServicesSupported	A bit string of which BACnet services the device supports
Protocol_Object_Types_Supported	BACnetObjectTypesSupported	A bit string of which object types the device supports
Object_List	BACnetARRAY[N] of BACnetObjectIdentifier	Every object the device contains
Max_APDU_Length_Accepted	Unsigned	The largest message the device accepts in one exchange
Segmentation_Supported	BACnetSegmentation	Whether the device can split large messages across packets, and in which direction
APDU_Timeout	Unsigned	How long the device waits for a reply before retrying, in milliseconds
Number_Of_APDU_Retries	Unsigned	How many times the device retries a request
Device_Address_Binding	BACnetLIST of BACnetAddressBinding	The device’s cache of other devices’ instance-to-address bindings
Database_Revision	Unsigned	A counter that changes when the device’s object database changes
Property_List	BACnetARRAY[N] of BACnetPropertyIdentifier	The list of properties this Device object implements

Object_List is often too large to read in one response; a client reads it by array index (index 0 gives the count) or uses ReadRange, which is why segmentation support matters. Reading Device 4194303 (the wildcard instance) with ReadProperty makes any device answer as its own Device object, the standard way to learn a device’s instance number.

Optional Properties

The Device object has a large optional set covering time, backup and restore, restart, and event reporting. The properties integrators meet most are Location and Description, the time properties (Local_Time, Local_Date, UTC_Offset), Max_Master and Max_Info_Frames on MS/TP devices, and Last_Restart_Reason.

Property	Datatype	Group
Location	CharacterString	General
Description	CharacterString	General
Serial_Number	CharacterString	General
Structured_Object_List	BACnetARRAY[N] of BACnetObjectIdentifier	General
Max_Segments_Accepted	Unsigned	Segmentation and APDU
APDU_Segment_Timeout	Unsigned	Segmentation and APDU
Max_Master	Unsigned	Segmentation and APDU
Max_Info_Frames	Unsigned	Segmentation and APDU
Local_Time	Time	Time
Local_Date	Date	Time
UTC_Offset	INTEGER	Time
Daylight_Savings_Status	BOOLEAN	Time
Time_Synchronization_Recipients	BACnetLIST of BACnetRecipient	Time
UTC_Time_Synchronization_Recipients	BACnetLIST of BACnetRecipient	Time
Time_Synchronization_Interval	Unsigned	Time
Align_Intervals	BOOLEAN	Time
Interval_Offset	Unsigned	Time
Configuration_Files	BACnetARRAY[N] of BACnetObjectIdentifier	Backup and restore
Last_Restore_Time	BACnetTimeStamp	Backup and restore
Backup_Failure_Timeout	Unsigned	Backup and restore
Backup_Preparation_Time	Unsigned	Backup and restore
Restore_Preparation_Time	Unsigned	Backup and restore
Restore_Completion_Time	Unsigned	Backup and restore
Backup_And_Restore_State	BACnetBackupState	Backup and restore
Last_Restart_Reason	BACnetRestartReason	Restart
Time_Of_Device_Restart	BACnetTimeStamp	Restart
Restart_Notification_Recipients	BACnetLIST of BACnetRecipient	Restart
Active_COV_Subscriptions	BACnetLIST of BACnetCOVSubscription	COV reporting
Active_COV_Multiple_Subscriptions	BACnetLIST of BACnetCOVMultipleSubscription	COV reporting
Status_Flags	BACnetStatusFlags	Intrinsic reporting
Event_State	BACnetEventState	Intrinsic reporting
Reliability	BACnetReliability	Intrinsic reporting
Event_Detection_Enable	BOOLEAN	Intrinsic reporting
Notification_Class	Unsigned	Intrinsic reporting
Event_Enable	BACnetEventTransitionBits	Intrinsic reporting
Acked_Transitions	BACnetEventTransitionBits	Intrinsic reporting
Notify_Type	BACnetNotifyType	Intrinsic reporting
Event_Time_Stamps	BACnetARRAY[3] of BACnetTimeStamp	Intrinsic reporting
Event_Message_Texts	BACnetARRAY[3] of CharacterString	Intrinsic reporting
Event_Message_Texts_Config	BACnetARRAY[3] of CharacterString	Intrinsic reporting
Reliability_Evaluation_Inhibit	BOOLEAN	Intrinsic reporting
VT_Classes_Supported	BACnetLIST of BACnetVTClass	Virtual terminal (legacy)
Active_VT_Sessions	BACnetLIST of BACnetVTSession	Virtual terminal (legacy)
Audit_Level	BACnetAuditLevel	Auditing
Auditable_Operations	BACnetAuditOperationFlags	Auditing
Tags	BACnetARRAY[N] of BACnetNameValue	Tagging and profile
Profile_Location	CharacterString	Tagging and profile
Deployed_Profile_Location	CharacterString	Tagging and profile
Profile_Name	CharacterString	Tagging and profile

Related Objects

• Network Port: carries the per-network address, port, and datalink settings the Device object summarizes.
• Analog Input and the other object types: the points a Device object’s Object_List enumerates.

What Chipkin Provides

The Device object, including the full identity, capability, time, and backup-and-restore property set, is implemented in the CAS BACnet Stack; it is the foundation every device built on the stack exposes. Talk to us about modeling your device’s points.

Related Pages

• BACnet
• BACnet Objects
• BACnet Device Instance
• BACnet I-Am Service
• BACnet Network Port Object
• What Happens When a BACnet Device Reboots
• ReinitializeDevice, Every Flavor and What Each Does
• Restart Cause Codes and BMS Recovery Patterns
• DM-RD-B: ReinitializeDevice