COV Limit

To determine the value of the COV limit, you must judge what level of control and response is needed in order for points to perform correctly. Consider the following guidelines when determining what COV limit to set for a point:

• For LAI and LAO points, the COV limit cannot be less than the slope of the point. For example, you cannot set a COV limit of 1/100th of a degree for a sensor with a slope of 1/10th of a degree.
• A COV limit that is set too high may result in programs and applications not receiving adequate updates of point values.
• A COV limit that is set too low may slow the system response by increasing the amount of information being transmitted over the communications trunk. A point with a COV limit that is too low may also toggle in and out of an alarm state resulting in an increased number of alarms that occur because the point momentarily exceeds its high limit or falls below its low limit.

The COV (Change-Of-Value) limit applies to LAI and LAO points. A COV limit defines the amount of change, in engineering units, a point (such as a temperature sensor) can experience before the field panel reports the change to other field panels and workstations.

When the field panel reads the value of the point, it compares it to the previously reported value for that point. If the difference between the last reported value and the current value is equal to or greater than the COV limit, the field panel reports the change to other field panels and workstations.