Alarm system, Terms

The usage of an alarm system in TwinCAT PLC Control obeys the following universal descriptions and definitions concerning alarms:

• Alarm: Generally an alarm is regarded as a special condition (expression value).
• Priority: The priority, also named "severity", of an alarm describes how important (severe) the alarm condition is. The highest priority is "0", the lowest valid priority value is "255".
• Alarm state: An expression/variable configured for the alarm control can have the following states: NORM (no alarm), INTO (alarm just has come), ACK (alarm has come and has been acknowledged by the user), OUTOF (alarm state has been terminated, alarm "has gone", but not yet acknowledged!)
• Sub-State: An alarm condition can have limits (Lo, Hi) and "extreme" limits (LoLo, HiHi). Example: The value of an expression ascends and first will transit the HI-limit, thus causing the coming of an HI-alarm. If the value continues ascending and exceeds also the HIHI-limit before the alarm gets acknowledged by the user, then the HI-alarm will get acknowledged automatically and just the HIHI-alarm remains in the alarm list (which is an internal list used for alarm administration). The HI-state in this case is named sub-state.
• Acknowledgement of alarms: The main purpose of alarms is to inform the user on alarm situations. In doing so it often is necessary to make sure that the user has noticed this information (see possible actions assigned to an alarm in the alarm class configuration). The user must acknowledge the alarm in order to get the alarm removed from the alarm list.
• Alarm Event: An alarm event must not be mixed up with an alarm condition. While an alarm condition can be valid for a longer period of time, an alarm event just describes the momentary occurrence of an change, e.g. a change from the normal state to the alarm state. In the TwinCAT PLC Control alarm configuration for the three types of events and the corresponding alarm states the same names are used (INTO, ACK, OUTOF).

In TwinCAT PLC Control the following features are supported:

• Deactivation of the alarm generation for single alarms as well as for alarm groups
• Selection of the alarms which should be displayed by defining alarm groups and priorities
• Saving of all alarm events in an alarm table
• Visualization element 'Alarm table' in the TwinCAT PLC Control Visualization