A deviation Alarmis commonly used to report an excess deviation between a desired set point level of a process value and an actual measurement of that value. The deviation Alarmbecomes active when the deviation exceeds or drops below a defined limit.

For example, if a set point had a value of 10, a high deviation Alarmlimit of 2 and a low deviation Alarmlimit of -1 then the low sub state is entered if the process value drops below 9; the high sub state is entered if the process value raises above 12. If the set point were changed to 11 then the new deviation values would be 10 and 13 respectively. The set point can be fixed by a configuration, adjusted by an Operatoror it can be adjusted by an algorithm, the actual functionality exposed by the set point is application specific. The deviation Alarmcan also be used to report a problem between a redundant data source where the difference between the primary source and the secondary source exceeds the included limit. In this case, the SetpointNodewould point to the secondary source.

The LowLimitand LowLowLimitshall be negative, indicating a number below the target value and the HighLimitand HighHighLimitshall be positive, indicating a number above the target value. If provided, the limits shall not be zero and shall follow these rules:

For example, if the LowLimitis -2 then a LowLowLimitof -1 would not be allowed, but a LowLowLimitof -3 would be allowed.