Using BACnet Trend Log Objects
In a BACnet network, a device is able to record changes in value of a property, locally within the physical BACnet device. The mechanism is often referred to as Trending.
Trending is managed by the Trend Log and Trend Log Multiple objects within the BACnet physical device. The object's properties define how trending is controlled.
- Trend recording at the device level. What, and how, properties are recorded.
- The conditions that lead to informing other devices that records are available for retrieval.
Frequently the Present Value property is used. Each value is recorded with a corresponding timestamp. Recording can be either polled, on change of value or triggered. Recording can behave either as a circular buffer with the oldest value overwritten if it becomes full, or as a fixed size buffer in which recording stops when it gets full. The Trend Log object can trigger a notification when the number of records reaches a particular value.
PcVue is able to retrieve the Trend records (values and timestamps) and store them in its own archive units. The retrieval can be executed periodically, as the result of a notification, or on-demand. The property that is recorded in the Trend Log object must also be mapped to a register variable. Once the data is saved in an archive unit it can be accessed in the same way as any other recorded trend data (displayed in a Trend Viewer, exported using Data Export or accessed using SCADA Basic etc.).
PcVue supports both Trend Log and Trend Log Multiple objects.