What is a Tag?

A tag in Ignition is essentially a named point of data, and may be a dynamic value that comes from an OPC address, an expression, or a SQL query, or it may be a static value. Tags also support scaling, alarming, and contain various meta data properties.

Tags provide a consistent data model throughout Ignition, and offer the easiest way to get up and running creating status, control, and history systems. Despite their low initial learning curve, however, tags offer a great amount of power in system design and configuration. Tags are held in tag providers, including potentially database based providers (SQLTagsTM), which can let you aggregate tags from a variety of installations together. User Defined Types (UDTs) provide an object-oriented approach to tag building, allowing you to define parameterized data types, extend and override types, and then rapidly generate instances. A change to the type definition is then inherited by all instances, drastically saving time when making routine changes. The UDT data types are fully supported by Vision templates, which means you can configure templates for your custom data types and take advantage of drag-and-drop binding to rapidly build complex screens.

For more information about the benefits of the Ignition tag system, see the Tags Overview in the Architecture chapter.

Tag Execution

Tags are executed by scan classes inside of a tag provider. In a typical system there will be one or two tag providers (the internal provider, which keeps the tag configuration in the project, and possibly an external tag provider in which tag configuration and values are stored in a database), and a number of scan classes.

The scan class provides basic timing information for the tag, although the actual evaluation may depend on the type of the tag. For example, with OPC tags, values are subscribed at the scan class rate, and then are updated on the tag immediately when they come in from the subscription. A query tag, however, is only evaluated when the scan class executes.

SQLTags stored in an external provider will be available to all Ignition installations that have access to that database. One of the installations will be specified as the tag's driver. The driving system will have a copy of the scan class that it executes, which in turn evaluates the tag. The value will be stored to the database, and all of the other installations will be notified of the new value.

For more information about tag providers, see Tags in the gateway configuration section.

What can tags do?

Fundamentally, the primary goal of a tag is to represent a value in the Ignition system. The values can come from OPC, expressions, queries, etc., and can be used on screens, in transaction groups, and more. Additionally, tags provide a core set of features above and beyond simple values:

Depending on the specific type of tag, even more options may be available. In general, tags provide a common interface for tying together many types of data in Ignition.