Trend Log

A Trend Log object records the history of one property of one object. It stores timestamped readings in a circular buffer that clients read back later with ReadRange. Use it to keep a record of a value even when no workstation is online to collect it.

Typical uses

• Room temperature history
• Energy meter readings at fixed intervals
• Changes in a status or setpoint
• Trigger-based snapshots on demand
• Data a front end pulls overnight

Key properties

Property	What it holds
Log_DeviceObjectProperty	The object and property being logged.
Logging_Type	How readings are taken: at a set interval (polled), when the value changes by an amount (COV), or when a client triggers one.
Log_Buffer	The stored records. A client reads them with ReadRange, not ReadProperty.
Buffer_Size	The most records the buffer holds. Once full, the oldest are overwritten unless Stop_When_Full is set.
Total_Record_Count	How many records have ever been logged. Clients use it to see what is new.
Enable	Switches logging on or off. Start_Time and Stop_Time can also bound it.

How it behaves

The object is not commandable. When it reports intrinsic events, the standard pairs it with the BUFFER_READY algorithm, which notifies a client once a chosen number of new records has been logged since the last notice. The Client_COV_Increment property sets how large a change triggers a COV-type record.

With the CAS BACnet Stack

Enable it with the STACK_OPTION_T_VMT_TRENDING_VIEWING_AND_MODIFYING_TRENDS build option, which also enables STACK_OPTION_OBJECT_TYPE_TREND_LOG. Do not use BACnetStack_AddObject: add the object with BACnetStack_AddTrendLogObject, giving the logged object and property and the buffer size. Then choose the logging type with BACnetStack_SetTrendLogTypeToPolled, BACnetStack_SetTrendLogTypeToCOV or BACnetStack_SetTrendLogTypeToTriggered, and bound it with BACnetStack_SetTrendLogStartStopTime. The stack stores the logging properties itself, so a callback cannot override them. Register BACnetStack_RegisterCallbackGetSystemTime, or writes to the object abort. Logging a property held by another device is not supported. Add and read records with BACnetStack_InsertTrendLogRecord and BACnetStack_ReadTrendLogRecord. For BUFFER_READY, enable alarms and call BACnetStack_SetIntrinsicBufferReadyAlgorithm.

Related object types

Trend Log Multiple, Event Log, Analog Input

Properties

The properties ASHRAE 135-2024 clause 12.25 defines for this object type.

How to read the table:

• Conformance is what the standard asks of every object of this type: R is required, W is required and writable by a client, O is optional. A digit after the letter marks a condition in the standard that decides when the property is present or writable, such as support for intrinsic reporting.
• Present by default: yes means the stack presents the property as soon as you add the object; no means it is optional and you turn it on with BACnetStack_SetPropertyEnabled. This is the stack’s starting setup, so it can differ from the conformance code.
• Writable is whether a client can write the property by default. Change it with BACnetStack_SetPropertyWritable.
• not in profile means the stack does not offer this property for this object type.
• Value when your callback does not answer: “Held by the stack” means the stack stores and serves the value itself and does not ask your callbacks. Otherwise it is the value the stack answers when your callback declines or is not registered.

Property	Datatype	Conformance	Present by default	Writable	Value when your callback does not answer
Object_Identifier	BACnetObjectIdentifier	R	yes	no	Held by the stack.
Object_Name	CharacterString	R	yes	no	"Object <type>,<instance>", such as "Object 0,1" for Analog Input 1.
Object_Type	BACnetObjectType	R	yes	no	Held by the stack.
Description	CharacterString	O	no	no	""
Enable	Boolean	W	yes	yes	Held by the stack.
Start_Time	BACnetDateTime	O1,2	no	yes	Held by the stack.
Stop_Time	BACnetDateTime	O1,2	no	yes	Held by the stack.
Log_DeviceObjectProperty	BACnetDeviceObjectPropertyReference	O8	no	no	Held by the stack.
Log_Interval	Unsigned	O1,3	no	yes	Held by the stack.
COV_Resubscription_Interval	Unsigned	O	no	no	0
Client_COV_Increment	BACnetClientCOV	O	no	no	Held by the stack.
Stop_When_Full	Boolean	R	yes	no	Held by the stack.
Buffer_Size	Unsigned32	R	yes	no	Held by the stack.
Log_Buffer	BACnetLIST of BACnetLogRecord	R	yes	no	Held by the stack. Read it with ReadRange; a ReadProperty is refused.
Record_Count	Unsigned32	W	yes	yes	Held by the stack.
Total_Record_Count	Unsigned32	R	yes	no	Held by the stack.
Logging_Type	BACnetLoggingType	R	yes	no	Held by the stack.
Align_Intervals	Boolean	O5	no	yes	false
Interval_Offset	Unsigned	O5	no	yes	0
Trigger	Boolean	O	no	no	Held by the stack.
Status_Flags	BACnetStatusFlags	R	yes	no	Held by the stack.
Reliability	BACnetReliability	O	no	no	0
Notification_Threshold	Unsigned32	O4,7	no	no	0
Records_Since_Notification	Unsigned32	O4,7	no	no	0
Last_Notify_Record	Unsigned32	O4,7	no	no	0
Event_State	BACnetEventState	R	yes	no	NORMAL, or the alarm engine’s state once alarms are enabled.
Notification_Class	Unsigned	O4,7	no	no	0
Event_Enable	BACnetEventTransitionBits	O4,7	no	no	An empty bit string.
Acked_Transitions	BACnetEventTransitionBits	O4,7	no	no	An empty bit string.
Notify_Type	BACnetNotifyType	O4,7	no	no	0
Event_Time_Stamps	BACnetARRAY[3] of BACnetTimeStamp	O4,7	no	no	No default: the read fails.
Event_Message_Texts	BACnetARRAY[3] of CharacterString	O7	no	no	""
Event_Message_Texts_Config	BACnetARRAY[3] of CharacterString	O7	no	no	""
Event_Detection_Enable	Boolean	O4,7	no	no	false
Event_Algorithm_Inhibit_Ref	BACnetObjectPropertyReference	O7	no	no	No default: the read fails.
Event_Algorithm_Inhibit	Boolean	O7,9	no	no	false
Reliability_Evaluation_Inhibit	Boolean	O10	no	no	false
Audit_Level	BACnetAuditLevel	O11	not in profile	not in profile	No default: the read fails.
Auditable_Operations	BACnetAuditOperationFlags	O11	not in profile	not in profile	No default: the read fails.
Property_List	BACnetARRAY[N] of BACnetPropertyIdentifier	R	yes	no	Held by the stack.
Tags	BACnetARRAY[N] of BACnetNameValue	O	no	no	No default: the read fails.
Profile_Location	CharacterString	O	no	no	""
Profile_Name	CharacterString	O	no	no	""