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BACnet: The Trend Log Object

The Trend Log object is defined by the standard as a property monitor for a specific object. When certain conditions are reached, a log is produced with the property value and a date/time stamp which is placed into a buffer for future retrieval. This buffer can be optionally fixed in size by the developer.

The Trend Log object offers the developer two ways in which the value is logged:

  • Periodically
  • During a change in value

The log entry is referred to by the standard as a Trend Log Record. Other items included into the Trend Log Record:

  • Data Entry Error(s)
  • Logging Operational Status

The following table will present the actual properties and datatypes of the Trend Log object from the BACnet standard:

PropertyDatatype
Object_IdentifierBACnet Object Identifier
Object_NameCharacter String
Object_TypeBACnet Object Type
DescriptionCharacter String
Log_EnableBoolean
Start_TimeBACnet Date Time
Stop_TimeBACnet Date Time
Log_Device Object PropertyBACnet Device Object Property Reference
Log_IntervalUnsigned
COV_Resubscription_IntervalUnsigned
Client_COV_IncrementBACnet Client COV
Stop_When_FullBoolean
Buffer_SizeUnsigned32
Log_BufferList of BACnet Log Record
Record_CountUnsigned32
Total_Record_CountUnsigned32
Notification_ThresholdUnsigned32
Records_Since_NotificationUnsigned32
Last_Notify_RecordUnsigned32
Event_StateBACnet Event Event State
Notification_ClassUnsigned
Event_EnableBACnet Event Transition Bits
Acked_TransitionsBACnet Event Transition Bits
Notify_TypeBACnet Notify Type
Event_Time_StampsBACnet Array (3) of BACnet Time Stamp
Profile_NameCharacter String

The automation developer should pay close attention to the defined properties in the following list:

  • Log_Enable property – Enable/disable logging process.
  • Start_Time property – Start logging process on a specific date and time.
  • Stop_Time property – Stop logging process on a specific date and time.
  • Record_Count property – By indicating “0” will clear the buffer.
  • Log_Interval property – Specifies the logging interval.
  • Stop_When_Full property – Stop logging when the buffer is Full.
  • Buffer_Size property – Indicates the maximum number of records in the buffer.
  • Log_Buffer property – Details of the buffer itself.

The following example from the standard will present the Trend Log object structure from an actual automation application. This example will specifically focus on the record log of the air temperature in a certain room within a building:

Object_Identifier(Trend Log, Instance 1)
Object_Name“Room 3Log”
Object_TypeTREND_LOG
Description“Room 3 Temperature”
Log_EnableTRUE
Log_DeviceObjectProperty((Device, Instance 100), Analog Input, Instance 3, Present_Value)
Log_Interval6,000
Stop_When_FullFALSE
Buffer_Size250
Log_Buffer(((23-MAR-1998,12:32:33.0), 72.0,(FALSE,FALSE,FALSE,FALSE)),(23-MAR-1998,12:34:32.0),72.1,(FALSE,FALSE,FALSE,FALSE)),…)
Record_Count250
Total_Record_Count131040
Notification_Threshold83
Records_Since_Notification30
Last_Notify_Record131010
Event_StateNORMAL
Notification_Class1
Event_Enable{FALSE, TRUE, TRUE}
Acked_Transitions{TRUE, TRUE, TRUE}
Notify_TypeEVENT
Event_Time_Stamps((23-MAR-95, 18:50:21.2),(*-*-*,*:*:*.*),(23-MAR-95, 19:01:34.0))

Written by: Scott Cosby

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