Types of tags

There are several types of tags. While in discussing "tags" we commonly mean gateway executed tags, system and client tags can play an important role in the overall design of a project.

Gateway Executed Tags

Tags executed in the gateway support all of the primary features of tags in Ignition: scaling, alerting, history, scripting, and role based permissions. They are identical in their configurations, apart from defining how the value is generated.

OPC Tags

OPC tags specify an OPC server and address which drives their values. The OPC address will be subscribed at the rate of the tag's scan class.

Memory Tags

These tags are simply values. The value is specified during configuration, and is stored when written (if the tag allows writing).

Expression Tags

These tags are driven by an expression. The expression syntax is the same as for property bindings, and allows mathematical operations, references to other tags, logic operations and more.

SQL Query Tags

These tags execute a SQL Query, whose result provides the value for the tag. Like SQL binding in Vision, SQL Query tags can reference other tags to build dynamic queries.

Complex Tags (UDTs)

Complex tags are created out of standard tag types, but offer a variety of additional features. In simple terms, you can think of them as a way to create "data templates", where a particular structure of tags is defined, and can then be created as if it were a single tag.

System Tags

System tags provide status about the system, such as memory usage, performance metrics, etc. They exist for the client and the gateway. Gateway system tags can be modified by the user to use alarming, history, and scaling, while client tags cannot.

Client Tags

Client tags, as the name implies, are only available for use in clients. This means that their values are isolated to a client runtime, and even though they are created in the designer, each client will create their own instances. This makes them very useful as in-project variables, for passing information between screens, and between other parts of the clients, such as scripting.

Client tags are a hybrid of memory, expression, and sql query tags. However, they do not have a scan class. When set to run as an expression or query, a poll rate is specified dictating how often the value should be calculated.