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
When such a condition exists, the alarm is "annunciated", usually both accoustically and
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
- 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
- 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.
- the assignment of threshold value(s) in case of an analog PV.
- 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:
- If the alarm handler subscribes to PVs which have no alarms configured, no alarms will be
posted by the IOC and therefore the alarm handler will never annunciate an alarm for these PVs. A
waste of alarm handler configuration!
- If an IOC has PVs configured for alarms, but no alarm handler subscribes to these PVs, the IOC
will see no need to post any alarms. Too bad for the IOC configuration effort!
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