Often, it is more efficient to process data after key events occur, such as the changing of a set point or the closing of a contact. Processing that is triggered by events rather than time is known as exception-based processing.
Exception-based processing is essential for true distributed SCADA applications that monitor a large number of I/O devices. For example, an oil field may be monitoring pipelines through a large network of remote terminal units (RTUs). The data from the RTUs changes infrequently, so there's no need to acquire data at a fixed interval. However, when the data does change, oil field operators need to know about it immediately. iFIX senses the changed value and processes it immediately.
SAC can perform exception-based processing on the following events:
- Data changes in the database.
- Unsolicited messages from the process hardware.
- Operator actions.
- Instructions from software applications.
