Modes and Setpoints
Each alarm is configured with one of many Modes, and usually one or more Setpoints. Modes determine the method in which alarm activity is evaluated, while Setpoints are the thresholds or limits that determine when a Tag is within the alarm state. Both properties work in together to determine when alarms become active, as well as cleared.
For example, when Mode is set to Below Setpoint, the Setpoint attribute is compared to the Tag's value. If the Tag value is less than the Setpoint value, the alarm becomes active. Once the value of the Tag is greater than the Setpoint value, the alarm will transition to a cleared state.
Alarms in Transaction Groups
Alarms can also be added to OPC items in Transaction Groups. This means alarms can be used without ever creating a Tag in Ignition. Simply edit an OPC item, and an Alarming section will appear on the side of the Edit Group Tag window. From here, adding an alarm to the item is similar to adding an alarm to an Ignition Tag.