The following function blocks can be alarm sources:

  • Analog Input / Analog Output / Analog Value
  • Binary Input / Binary Output / Binary Value / Pulse Converter
  • Multistate Input / Multistate Output / Multistate Value
  • Event enrollment
  • Command Control object2
  • Power Control object2
  • Schedulers (Analog / Binary / Multistate Scheduler object)2
  • AlarmCollection object
  • Discipline I/O1, 2
  • Trend Log / Trend Log Multiple
  • Group1, 2
  • Device Info object, which models the properties of an automation station as a complete entity
  • Loop object



Discipline I/Os, Groups, Time Scheduler and Trend Log Multiple support only system alarms, that is, only alarms of the FAULT type. Both function blocks can transmit more than one system alarm. The parameters [Rlb] and [MsgTxt] provide detailed information about the cause of the most recent alarm message. The messages are transmitted in the order in which they occur, irrespective of the importance of the alarm.


These function blocks only exist in Desigo PX.

Only these alarm sources incorporate intrinsic reporting, and can thus generate their own alarms. If any other value of a function block needs to be monitored for an alarm (e.g., the control signal for a controller block), an Event Enrollment object must be added.

Alarm-generating function blocks include a range of interface variables which can be set as parameters to determine the alarm response (Input Property) or to supply the relevant alarm state information (Output Property). These interface variables are described further below. Some of the interface variables are common to all alarm-generating block types, while others are specific to certain types of alarm-generating blocks.

Alarm state machine in an alarm-generating function block

Alarm state machine

The response in the event of an alarm is modeled by an alarm state machine. Each alarm-generating block incorporates an alarm state machine of this type. The alarm-related interface variables can therefore be used to define the response of this state machine, to simulate state transitions, or to represent the current status of the state machine itself.

Alarm state event states

The alarm state machine can assume one of three basic states (event states [EvtSta]):

  • NORMAL: There is no alarm condition present
  • OFFNORMAL: Alarm caused by an OFFNORMAL condition
  • FAULT: Alarm caused by a FAULT condition

With analog blocks, the OFFNORMAL state is explicitly subdivided into the sub-states HIGH LIMIT and LOW LIMIT, which are described in detail further below.

The current state of the alarm state machine in an alarm-generating block is displayed externally in the form of the output variable [EvtSta] (event state) of the block concerned.

State transitions between alarm states



Action / Event state


A new OFFNORMAL alarm condition has been detected.



The current OFFNORMAL alarm condition has disappeared, and there is no other alarm condition present.



A new FAULT alarm condition has been detected.



The current FAULT alarm condition has disappeared, and there is currently no other alarm condition.


System events may also occur within each alarm state. These message functions do not affect the alarm state.

Because FAULT alarms take priority over OFFNORMAL alarms, the state transition from FAULT to OFFNORMAL only occurs under very special circumstances.

If, while in the OFFNORMAL state, a FAULT alarm condition occurs, there is then a state transition TO_FAULT (because as stated above, FAULT takes priority over OFFNORMAL).