The Trend Log object monitors and records the changes in the behavior of an individual attribute over time, thereby assisting you with diagnosing various system-wide behavioral characteristics. For example, use a Trend Log to collect data from the Analog Input object of a field controller, which reports information such as outdoor air temperature or room air temperature.
The trend log collects sample values at timed intervals or only upon changes in the given value. Trend data is still collected if the object is unreliable. The trend log continues to log data using the previous value preceded by ???, which indicates an unreliable status.
You cannot define a Trend extension under the Trend Log object like you can with other objects. As a requirement for adding a trend log object from the UI, you select an input reference, then enable the trend log so that it can begin to collect samples.
Trend Log samples are stored locally with a default buffer size of 144 and a maximum of 5000 samples. Unlike a Trend extension on an object, repository storage is not available with the Trend Log object. The intention is that data sampled by the trend log object is stored at the BACnet client.
For general information on Metasys system objects, see Common Object Attributes.