Alarm tags
In the PLC, dedicated tags or data block addresses are required to be used in order to tell the HMI that an alarm has been raised. These exist in the HMI as the trigger tag and acknowledgment status tag.
Logic is developed against these tags in order to react to operator acknowledgment of alarms and to raise the instance of the alarm. Figure 12.19 shows a basic example of ladder logic controlling a latching interlock:
Figure 12.20 – An example of a basic alarm interface
When Scaled_Value for Instrument_1 is above High_Alarm_Trigger, both the trigger tag ("Alarms".Alarm_Group_1.Active[0].%X0) and High_Interlock are set to True. When this occurs, the HMI writes to the acknowledgment status tag ("Alarms".Alarm_Group_1.Accepted[0].%X0) and sets it to False, which then holds on to the trigger tag:
Figure 12.21 – A latched alarm in the HMI
If the trigger tag returns to a False value, the acknowledgment...