Tag Extensions

System Model > Variable Tags > Tag Extensions

Tag extensions allow you to refer to the elements of a tag. They support the following functionality:

Provide access to the quality and time stamp information associated with a tag value.
Provide extended data to Citect SCADA client components, such as a display client, trend server, alarm server, report server, Cicode and CtAPI.
Allow you to apply an override to a tag, and prohibit writing to the tag value.
Allow you to display and trend tag values even if their quality is not "good".
Enable tag value persistence, and replication of tag data.

The tag reference syntax used for tag extensions is as follows:

[Cluster.]Tag[.Element][.Item][ [n]]

Where:

Cluster	The optional cluster name.
Tag	The tag name or SuperGenie association.
Element	The optional element name. If the element name is not specified, the requested element will be determined at runtime.
Item	The optional item name. If the item name is not specified, the whole element is referenced.
n	The optional array index if the tag is defined as an array.

The array index is at the end of the reference (MyArray.v[n], MyArray.Field[n], MyArray.Field.v[n]). There is only a single quality and timestamp for each array, each member will return the same quality and timestamp.

Note: Consider the impact on network traffic when configuring tag extensions, as the distribution of quality and value timestamps increases the amount of data being sent between servers.

You can use tag extensions to access tag data in the following ways:

Reference tag data using only the tag name, for example:

"MyTag" (unqualified tag reference).

This will provide default access to the .field element information, unless the tag is in one of the override modes.

Reference tag data using the tag name and the item name, for example:

"MyTag.q" (unqualified tag reference)

This will provide access to the item information for the tag, either default from .field or .override element.

Reference the tag data by using the tag name and the element name, for example:

"MyTag.field" (qualified tag reference)

This reference will provide access to the specified tag element information.

Reference the tag data by using the tag name, element name and the item name, for example:

"MyTag.field.vt" (qualified tag reference)

This reference will provide access to the specific tag element item.

You can also access alarm data in a similar way. See Using Alarm Properties as Tags.

Note: Consider the impact on network traffic when configuring tag extensions, as the distribution of quality and value timestamps increases the amount of data being sent between servers.

Controlling Tag Extension Behavior

By default, the tag data referenced without an element will provide access to the data value when the value is of quality is good and an error (#BAD, #COM, etc) when the quality is bad. The configuration parameter [Page]IgnoreValueQuality can be used to change this behavior, including automatically changing the background color of text and number graphics objects on a page with changes in quality of the tag.

Tag extension behavior is controlled by several citect.ini file settings which are described in the Parameters Reference. For each of these settings there is a corresponding setting in the project database parameters (param.dbf).

A citect.ini file setting specifies behavior for a particular machine and a parameter database setting is applied system-wide (see About Parameters).

Note: By default, the TagSubscribe Cicode function is set to retrieve "lightweight" tag values that exclude quality and value timestamps. If you need a subscription to retrieve timestamp data, you need to set the "bLightweight" argument to 0 (false).