Read Tag

Returns the data value stored in a tag.

1378

Inputs/Outputs

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tag/proxy ref in

The reference to the tag endpoint or tag proxy endpoint from which you want to read the data value.

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data type

Data type you input for the tag that the tag endpoint reads.

To prevent the node from returning an error if the node cannot determine the data type of the tag at run time, wire a value to this input. Otherwise, you can ignore this terminal because the data type of this input automatically changes to match the data type of the tag to which it is wired.

Default value: 0

Purposes of data type

This terminal has two purposes:
  • If the compiler cannot determine the type of the tag automatically, you can manually specify the type by wiring this terminal.
  • You want to pre-allocate the memory for the tag at this point. The read operation attempts to reuse any buffers associated with the value you wire to this terminal. Minimizing memory allocations for each read operation can improve performance.
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error in

Error conditions that occur before this node runs.

The node responds to this input according to standard error behavior.

Standard Error Behavior

Default value: No error

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tag/proxy ref out

The reference to the tag endpoint or tag proxy endpoint from which this node reads the data value.

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value

The data value stored in the tag.

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timestamp

The timestamp associated with the data value.

This output is available only if all of the following conditions are met:

  • When you create the tag, you enable the Has Timestamps? property.
  • You wire a reference to that tag to tag/proxy ref in.

Associating Timestamps with Tag Data

Complete the following steps to associate a timestamp with the data value stored in a tag.

  1. Navigate to the shared resource collection document where you created the tag and select the tag you want to modify.
    Note Shared resource collection documents have the .grsc file extension.
  2. On the Item tab, enable the Has Timestamps? checkbox.

If the tag contains an initial value, the tag now associates that value with the timestamp when the tag was created.

You can also associate a custom timestamp to the data value that you write to a tag. If you wire a timestamp to the Write Tag node when writing a new data value to a tag, the node then associates that timestamp with the new data value.

Note Associating a timestamp with tag data may slow the performance of your application. Associate timestamps with tag data only when necessary.
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error out

Error information.

The node produces this output according to standard error behavior.

Standard Error Behavior

What Is a Tag?

A tag is single-point, latest value data that you can access from multiple documents within a project. A tag can store a single value of any data type that represents the state of a process. Tags are similar to global variables in text-based programming languages.

You can create a tag either dynamically, using the Configure and Create Tag node on the diagram, or statically, in the shared resource collection file.

You can create a tag to access the same data from multiple VIs within a project.

What Is a Tag Proxy?

A tag proxy is a reference that is local to part of the application that mirrors the value of the actual tag to which it is linked in another location in the application. You use a tag proxy to access data that is in another location in the application. You can read from and optionally write to a tag proxy only after the tag proxy links to a tag.

Using a Tag Constant

Complete the following steps to statically create and configure a tag.

  1. Create or open a shared resource collection document.
    Note Shared resource collection documents have the .grsc file extension.
  2. Expand the Tag section and click Create New.
  3. Configure the data type, initial value, and other properties of the tag in the Item tab.