The Primary Normal Phase of a System Scan is used to retrieve the current data of configured points from connected outstations. During the Primary Normal Phase, current data is retrieved from every connected outstation, except for those outstations that:
- are configured with individual polling intervals and offsets
and
- are not due to be scanned by the driver at that particular time (see Specify the Polling Intervals and Offset).
Outstations are scanned for primary data in ascending outstation number order starting from the lowest outstation address. During the Primary Normal Phase, the driver will only visit an outstation once for extracting data. However, a number of protocol exchanges may take place per visit.
The Primary Normal Phase runs until it has scanned the appropriate outstations for current data. In exceptional circumstances, this may cause the Primary Phase to exceed the System Scan time (see System Scan Interval). For example, the Primary Phase may exceed the System Scan time when system is first started (see Establishment Phase).