Configure deviation alarms

A deviation alarm is associated with integer or real tags. You can set an alarm by comparing the current tag value to a target value, and then the absolute value of the difference is compared to one or more limits, expressed as a percentage of the range of the tag value.

For example, the following values set the conditions for a tag’s minor and major deviation alarms:

Minimum Value = -1000

Maximum Value = 1000

Minor Deviation % = 10

Major Deviation % = 15

Target Value = 500

Using these values as an example, the minor and major deviation alarms points are calculated by the following steps:

1. Calculate the total value range of the tag.

   1000 - (-1000) = 2000

2. Multiply the total value range of the tag by minor and major deviation percentages.

   2000 x 0.10 = 200 = minor deviation limit

   2000 x 0.15 = 300 = major deviation limit

3. Add and subtract the minor and major deviation limits from the target value.

   500 - 200 = 300 = minor deviation lower limit

   500 + 200 = 700 = minor deviation upper limit

   500 - 300 = 200 = Major deviation lower limit

   500 + 300 = 800 = Major deviation upper limit

Configure a deviation alarm

4. Open the Tagname Dictionary.

5. Select an existing real or integer tag or create a new tag.

6. Select either Alarms or Details & Alarms at the top of the Tagname Dictionary dialog box to show the alarm details dialog box.

   

7. Select the deviation (Minor and Major Deviation) alarm types you want to use to detect when the value of an analog type tag is in a major or minor deviation from the specified target value.

8. In the %Deviation box, type the percentage that the analog tag can deviate from the target value to trigger a minor or major deviation alarm condition. It is expressed as a percentage of the range of the tag. For an I/O tag, the Min EU and Max EU values entered in the tag's details dialog box define the range. For memory tags, the range is defined by the minimum value and maximum value.

9. In the Target box, type the tag reference value that minor and major deviation percentages are based.

10. In the Deviation Deadband % box, type the deviation percentage the tag value must drop below the limit before the tag is taken out of its alarm condition.

11. Select Save.

12. Select Close to close the Tagname Dictionary dialog box.