BACnet - What is a BACnet Trend Log object
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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:
| Property | Datatype |
| Object_Identifier | BACnet Object Identifier |
| Object_Name | Character String |
| Object_Type | BACnet Object Type |
| Description | Character String |
| Log_Enable | Boolean |
| Start_Time | BACnet Date Time |
| Stop_Time | BACnet Date Time |
| Log_Device Object Property | BACnet Device Object Property Reference |
| Log_Interval | Unsigned |
| COV_Resubscription_Interval | Unsigned |
| Client_COV_Increment | BACnet Client COV |
| Stop_When_Full | Boolean |
| Buffer_Size | Unsigned32 |
| Log_Buffer | List of BACnet Log Record |
| Record_Count | Unsigned32 |
| Total_Record_Count | Unsigned32 |
| Notification_Threshold | Unsigned32 |
| Records_Since_Notification | Unsigned32 |
| Last_Notify_Record | Unsigned32 |
| Event_State | BACnet Event Event State |
| Notification_Class | Unsigned |
| Event_Enable | BACnet Event Transition Bits |
| Acked_Transitions | BACnet Event Transition Bits |
| Notify_Type | BACnet Notify Type |
| Event_Time_Stamps | BACnet Array (3) of BACnet Time Stamp |
| Profile_Name | Character 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_Type | TREND_LOG |
| Description | "Room 3 Temperature" |
| Log_Enable | TRUE |
| Log_DeviceObjectProperty | ((Device, Instance 100), Analog Input, Instance 3, Present_Value) |
| Log_Interval | 6,000 |
| Stop_When_Full | FALSE |
| Buffer_Size | 250 |
| 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_Count | 250 |
| Total_Record_Count | 131040 |
| Notification_Threshold | 83 |
| Records_Since_Notification | 30 |
| Last_Notify_Record | 131010 |
| Event_State | NORMAL |
| Notification_Class | 1 |
| Event_Enable | {FALSE, TRUE, TRUE} |
| Acked_Transitions | {TRUE, TRUE, TRUE} |
| Notify_Type | EVENT |
| Event_Time_Stamps | ((23-MAR-95, 18:50:21.2),(*-*-*,*:*:*.*),(23-MAR-95, 19:01:34.0)) |