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Tag Type

The types of tag in WAGO SCADA include:

  • Memory Tag
  • I/O Tag
  • Expression Tag
  • System Tag

Of these, memory tags, I/O tags, and expression tags are allowed to be created, while system tags are built-in and do not support creation.

The types of tags that can be created by the user have their own icons in the asset tree:

Icon Tag Type
alt text Memory tag
alt text I/O tag
alt text Expression tag

Memory Tag

Memory tag are simple tags that are not polled or updated automatically. They remain the same value until some mechanism created by another user (perhaps a script or binding) changes their value.

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I/O Tag

Obtain their values by binding to an external device.

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I/O tags get their values by binding to a device through a data source property.

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Expression Tag

Calculates and determines its value by writing an expression.

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Expression tags must have the Data Source Path, Expression (Read), and Expression (Write) set.

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Name Description
Data Source Path You can enter the tag Path directly or select the tag. Only one tag can be filled in.
Expression (Read) The default display {Source}, {Source} represents the value of the source tag. You can use the new value obtained from the source tag by expression calculation as the value read by the current tag.
Expression (Write) The default display {Value}, {Value} represents the original value of the source tag. This expression determines the value that should be written to the current tag.

Example

To convert degrees Fahrenheit to Celsius for display.

  1. Create an expression tag with the name: Celsius_Temperature
  2. Set the source tag, select the source tag "Fahrenheit_Temperature", the value of this tag is a Fahrenheit temperature.
  3. Set the Expression (Read) to: ({Source} - 32) × 5/9
  4. Set the Expression (Write) to: ({Value} × 9/5) + 32