Alarm Concepts 


This sections describes alarm concepts in the context of an EPICS-based control system. Some familiarity with EPICS control system topology and terminology is assumed.


An alarm is a notification issued by the control system to a user or operator about a  condition in the control system which needs a human's attention.

When such a condition exists, the alarm is "annunciated", usually both accoustically and optically.
Again usually, but not necessarily, the annunciation is performed by a specialized Channel Access client, which subscribes to alarm notifications from one or more IOCs.
The TAlh alarm handler is such a client.

The condition in the control system is based on properties of a process variable (PV).  Two separate configuration steps are required so that the annunciation of the condition can happen:
  1. in the IOC which "owns" the process variable (PV) , such as an analog readback or a binary state, the PV must be configured to post alarms. This configuration is usually done by the controls group or the operations group. The configuration involves 
    1. the assignment of an "alarm severity" to a process variable state
      state examples:  on, off, change of state, above a threshold, below a threshold
      severity levels:  no alarm, minor, major, invalid    - this set of severities is fixed in an EPICS system.
    2. the assignment of threshold value(s) in case of an analog PV.

  2. The alarm handler must be configured to subscibe to the alarms posted by PV. This is also called the creation of an "Alarm Channel".
It is important that both configuration steps are synchronized:
Note that the alarm state of a PV is available to any Channel Access client by subscribing to the PV's properties.


From an operator's point of view Monitors are similar to alarms, but they do not involve any IOC configuration. They are configured in the TAlh instance.

Note that the state of  a PV monitor is only available to the TAlh instance, not to other Channel Access clients.

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