Applies toMasking is normally a temporary operation connected with the current state of the process being monitored. An alarm that has any mask other than 0 continues to be scanned but its value will not be propagated outside the real time database. Hence any activities connected to that alarm will stop. This includes alarming and recording to disk. The value is still accessible however, by a SCADA BASIC program. The mask of an alarm may be used and cleared from several sources, each represented by a binary weight in the mask:
- By a SCADA BASIC program. Up to four levels of mask may be set in this way:
1 = Level 1
2 = Level 2
4 = Level 3
8 = Level 4
- By the operator. For example, an alarm bit may be masked using an option available from the Alarm Viewer. The binary weight of this option is 16.
- By dependence. If the value of a variable depends on another, for example a threshold bit, the source variable going invalid will cause the dependent variable mask to be set causing it also to be invalid. The binary weight of this option is 32. Another example of an alarm that may appear as masked is an alarm that is inhibited by another alarm or bit.
When an alarm is unmasked, its value immediately becomes available.