BACnet Trend Log Object

What the Trend Log Object Is

The Trend Log object, defined in Clause 12.25 of ANSI/ASHRAE 135-2024, is how a BACnet device records the history of a single property over time. It watches one referenced property, and when a logging condition is met it saves the value and a timestamp into an internal buffer. The referenced property can live in the same device or in another device on the network.

Log_DeviceObjectProperty names the property being logged, and Logging_Type sets how samples are taken: periodically on Log_Interval, on a change of value via COV (Change of Value), or when triggered by a write to the Trigger property. The buffer is read back with the ReadRange service, and writing zero to Record_Count clears it. When a Trend Log supports intrinsic reporting it applies the BUFFER_READY event algorithm, sending a notification once Notification_Threshold new records have accumulated so a head end (the central BMS server) knows to fetch them.

Where You’ll See It

• Sensor history. A Trend Log records a space temperature or flow rate at a fixed interval so operators can review performance and diagnose comfort complaints after the fact.
• COV-driven logging. For values that change irregularly, a Trend Log subscribes for COV and stores only the changes, keeping the buffer compact.
• Automatic collection. With intrinsic reporting enabled, the Trend Log signals a fileserver or head end through BUFFER_READY, which then pulls the new records with a ReadRange request.

Required Properties

Clause 12.25 requires every Trend Log object to provide these properties. Datatypes are the spec’s names: BOOLEAN is a true/false flag, Unsigned an integer, BACnetLIST of BACnetLogRecord is the timestamped buffer, and the other BACnet-prefixed types are structured values defined by the standard.

Property	Datatype	What it holds
Object_Identifier	BACnetObjectIdentifier	The object’s type and instance number, unique within the device
Object_Name	CharacterString	The human-readable point name, unique within the device
Object_Type	BACnetObjectType	Always trend-log for this object
Enable	BOOLEAN	Writable; turns logging on and off
Stop_When_Full	BOOLEAN	When true, logging stops once the buffer fills instead of overwriting
Buffer_Size	Unsigned	The maximum number of records the buffer holds
Log_Buffer	BACnetLIST of BACnetLogRecord	The stored timestamped records, read with ReadRange
Record_Count	Unsigned	Writable; the number of records in the buffer, and writing zero clears it
Total_Record_Count	Unsigned32	The running total of records ever collected, used as the sequence number. Monotonic: it does not decrease on wrap, and rolls over from 4294967295 to 1
Logging_Type	BACnetLoggingType	Whether sampling is polled, COV-driven, or triggered
Status_Flags	BACnetStatusFlags	Four health flags: in-alarm, fault, overridden, out-of-service
Event_State	BACnetEventState	The object’s current event state. It stays normal for buffer-ready reporting, which is dispatched as a TO-NORMAL transition from NORMAL. See BUFFER_READY
Property_List	BACnetARRAY[N] of BACnetPropertyIdentifier	The list of properties this instance actually implements

Optional Properties

The properties integrators meet most are Log_DeviceObjectProperty (the property being logged), Log_Interval (how often, for periodic logging), and Start_Time and Stop_Time (the window during which collection runs). The intrinsic-reporting group appears or is absent as a set.

Property	Datatype	Group
Description	CharacterString	General
Log_DeviceObjectProperty	BACnetDeviceObjectPropertyReference	Logging
Log_Interval	Unsigned	Logging
Start_Time	BACnetDateTime	Logging
Stop_Time	BACnetDateTime	Logging
COV_Resubscription_Interval	Unsigned	Logging
Client_COV_Increment	BACnetClientCOV	Logging
Align_Intervals	BOOLEAN	Logging
Interval_Offset	Unsigned	Logging
Trigger	BOOLEAN	Logging
Reliability	BACnetReliability	Fault detection
Notification_Threshold	Unsigned	Intrinsic reporting
Records_Since_Notification	Unsigned	Intrinsic reporting
Last_Notify_Record	Unsigned	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
Event_Detection_Enable	BOOLEAN	Intrinsic reporting
Event_Algorithm_Inhibit_Ref	BACnetObjectPropertyReference	Intrinsic reporting
Event_Algorithm_Inhibit	BOOLEAN	Intrinsic reporting
Reliability_Evaluation_Inhibit	BOOLEAN	Fault detection
Audit_Level	BACnetAuditLevel	Auditing
Auditable_Operations	BACnetAuditOperationFlags	Auditing
Tags	BACnetARRAY[N] of BACnetNameValue	Tagging and profile
Profile_Location	CharacterString	Tagging and profile
Profile_Name	CharacterString	Tagging and profile

Related Objects

• Trend Log Multiple: logs several properties in lockstep instead of one.
• Event Log: records event notifications rather than property values.
• Notification Class: routes the BUFFER_READY notification to its recipients.

What Chipkin Provides

The Trend Log object, including periodic, COV, and triggered logging and the BUFFER_READY intrinsic reporting algorithm, is supported in the CAS BACnet Stack and hosted by B-BC building controllers. Talk to us about modeling your device’s points.

Related Pages

• Why “It Works on My Bench” Is Not BTL Conformance
• BACnet
• BACnet Objects
• BACnet ReadRange Service
• BACnet Trend Log Multiple Object
• BACnet Object Types & Properties Reference
• BACnet Total_Record_Count vs Record_Count: what the counters mean, what actually loses history, and reading a buffer that wraps
• BACnet BUFFER_READY: The Event That Fires Without a State Change: why the notification silently never arrives