Trend sampling for the trend buffer occurs periodically (samples taken after a set time expires) if you set the Sample Interval to a value higher than zero. To sample based on COV reporting of the object, set the Sample Interval attribute to zero. You can stop or start sampling manually (using the Enable or Disable command) or control the start and stop with Interlocking a Trend Extension or scheduling. With interlocking, the trend samples are taken when a defined event occurs. If you use the scheduling feature, trend samples are taken at certain times on certain days.