Tags and the project database

Tags are a core component of any project. Simply put, project tags are variables used to receive and store data obtained from communication with plant floor devices, from the results of calculations and functions, and from user input. In turn, tags can be used to display information on screens (and Web pages), to manipulate screen objects, and to control runtime tasks.

But tags are more than simple variables. The project runtime includes a real-time database manager that provides a number of sophisticated functions such as time-stamping of any value change, checking tag values against runtime minimum and maximum values, comparing tag values to alarming limits, and so on. A project tag has both a value and various properties that can be accessed, some at development and others only at runtime.

All tags are organized into one of the following categories, which are represented by folders on the Global tab of the Project Explorer:

• Project Tags folder are tags that you create during project development. Places where project tags are used include:

  • Screen tags

  • Tags that read from/write to field equipment

  • Control tags

  • Auxiliary tags used to perform mathematical calculations

• Classes are compound tags composed of members (basic tags).

• Shared Database tags are created in a PLC program and then imported into the tags database through the Tag Integration feature.

  For example you might create tags in CoDeSys and then import them into your project so that it can directly read/write data on a CoDeSys device.

  You cannot modify the properties of shared tags within your project. Instead, modify those tags in the original PLC program and then re-import them into the tags database.

• System Tags are predefined tags with predetermined functions that are used for supervisory tasks during project run time. For example,

  • Date tags hold the current date in string format

  • Time tags hold the current time in string format

    Most system tags are read-only, which means you cannot add, edit, or remove these tags from the database.

    To see a list of the system tags, select the Global tab in the Project Explorer, open the System Tags folder, and open the Tag List subfolder.

After creating a tag, you can use it anywhere within the project, and you can use the same tag for more than one object or attribute.