Understanding Shared Variable Alarming Data Items (DSC Module)

When you enable alarming for a shared variable and deploy the shared variable, LabVIEW creates shared variable alarming data items in the Shared Variable Engine (SVE). The alarming data items reside in a folder named Alarms. This Alarms folder resides under the shared variable.

You use alarming data items to read from and write values to alarms. You can read and write alarming data items by using the NI Publish-Subscribe Protocol (NI-PSP) clients, such as the Shared Variable node, VI, and functions, the NI Distributed System Manager, and so on. To launch the System Manager from LabVIEW, select Tools»Distributed System Manager.

To read from or write a value to an alarming data item, you can specify the data item URL in the following format: \computer\process\variable\Alarms\alarm\data_item. If you use the Shared Variable Node, VI, and Functions to read and write alarming data items, specify the data item URL in the following format: ni.var.psp://computer/process/variable/Alarms/alarm/data_item. In both URL formats, variable stands for the name of the shared variable under which the data item resides and alarm stands for the alarm name you specify.

The default values of alarming data items are the values that you specify for the alarming property when you configure a shared variable. However, the values that you write to alarming data items do not change the values that you specify for the alarming property. If you restart the SVE or the computer, LabVIEW initializes the values you specify for alarming data items. The values that LabVIEW initializes are the same as the values you specify for the alarming property.

The following table lists the shared variable alarming data items that the LabVIEW Datalogging and Supervisory Control (DSC) Module supports.

Data Item	Data Type	Access Type	Description
Ack	Boolean value	Read	Specifies whether an alarm is acknowledged for a shared variable.
AckType	32-bit unsigned integer	Read/Write	Specifies how an alarm is acknowledged. A value of 0 indicates that LabVIEW acknowledges the alarm when the shared variable value returns to a normal state. When you specify 0 for AckType, the acknowledgement type of the alarm is Auto. A value of 1 indicates that LabVIEW does not acknowledge an alarm until the user acknowledges the alarm regardless of the alarm state. LabVIEW does not generate any new alarm instance before you acknowledge the existing alarm instance. When you specify 1 for AckType, the acknowledgement type of the alarm is User. A value of 3 indicates that LabVIEW does not acknowledge an alarm until the user acknowledges the alarm. LabVIEW generates a new alarm instance whenever the alarm changes from a normal state to an abnormal state. When you specify 3 for AckType, the acknowledgement type of the alarm is Each Occurrence. Note  If you specify 3 for AckType, LabVIEW may generate a large number of alarm instances before you acknowledge them.
AlarmState	Boolean value	Read/Write	Sets alarm conditions for Boolean shared variables. You use this data item to set the Boolean shared variable to be in alarm state when the value is either TRUE or FALSE. This data item exists when you enable a Boolean alarm.
deadband	64-bit floating-point number	Read/Write	Specifies the alarm deadband for a shared variable as a percentage of the full scale range. This data item exists when you enable a HI_HI alarm, HI alarm, LO alarm, or LO_LO alarm.
Description	String	Read	Specifies the description of the alarm.
Enable	Boolean value	Read/Write	Specifies whether an alarm is enabled for a shared variable.
level	64-bit floating-point number	Read/Write	Specifies the value, in engineering units, that evokes the alarm condition. This data item exists when you enable a HI_HI alarm, HI alarm, LO alarm, or LO_LO alarm. The level values of the numeric alarms conform to the following rule: HI_HI level>HI level>LO level>LO_LO level.
Log	Boolean value	Read/Write	Specifies whether alarm logging is enabled.
Priority	32-bit unsigned integer	Read/Write	Specifies the priority of the alarm. Enter a value between 1 and 1000, where 1000 is the highest priority.
Set	Boolean value	Read	Specifies whether an alarm is set for a shared variable.