Alarm Server Process

System Model > Alarms > Alarm Server Process

An alarm server process is responsible for evaluating the conditions that define an alarm.

The I/O server sends a notification to the alarm server process each time the value changes for a variable tag that is associated with an alarm. If a notification results in an alarm condition occurring, the alarm server process will generate an alarm.

An alarm server process evaluates the notifications it receives from an I/O server at a rate that is double that used by the I/O server to poll the variable tags (that is, the rate set by the Citect.ini parameter AlarmScanTime.

For information on how to add an alarm server process to a project, see the topic Add an Alarm Server Process.

A Citect SCADA system can have more than one alarm server process. Multiple alarm server processes may be required in the following circumstances:

Redundant systems – alarm servers can be paired and synchronized so alarm data remains online if a primary server becomes inactive (see Alarm Server Redundancy in the chapter on Redundancy)
Clustered systems – multiple alarm servers can be added to a distributed system that includes multiple clusters (see Clusters).

If required, you can also configure an alarm server to operate in Extended Memory mode. This allows the alarm server process to utilize memory beyond a 4GB limit. It does this by running the process in 64 bit.

Note: When operating in Extended Memory mode, an alarm server process does not support the following:
• ActiveX objects
• calls from CitectVBA
• Cicode functions or alarm expressions that call a DLL compiled in 32 bit.

Extended Memory mode enables efficient query handling under the following circumstances:

On systems with a large archive of historical alarm data
On high-capacity systems that can generate a large number of alarms.

To enable this mode of operation, you need to set the alarm server's Extended Memory property to TRUE (see Add an Alarm Server Process). You also need to recompile your project and restart the runtime system for the change to take effect.