Tagging Process Variables

You need to assign a variable tag to each I/O DeviceAn item of equipment that communicates with plant-floor control or monitoring equipment (sensors, controllers, and so on). The most common I/O devices are PLCs (programmable logic controllers); however, supports a wide range of I/O devices, including loop controllers, bar code readers, scientific analyzers, remote terminal units (RTUs), and distributed control systems (DCS). can communicate with any I/O device that has a standard communications channel or data highway. variable that CitectSCADA uses in your runtime system. To define your variable tags, you declare them in the variable tag database. The variable tag becomes a label, used to reference the data value of the I/O Device register at the specified address. Using labels has several benefits:

• You do not have to remember the address every time you want to use the variable. You use the tag name, which has to be logical and descriptive, and therefore less confusing.
• The address in the I/O Device is defined only once. If you change the address, you only need to update the variable tag definition, not every instance in your configuration.
• You can scale the raw data to an appropriate range in the same declaration.
• You can access extended data value, quality and timestamp information.

You need to define your variables as a specific data type. The most common variables supported by I/O Devices are digital and integer. CitectSCADA also supports real, string, byte, bcd, long, and longbcd data types.

After you have defined your variable tags, you can use them to:

• Display objects on a graphics pageA drawing (or image) that appears on a workstation to provide operators with control of a plant, and display a visual representation of conditions within the plant..
• Store data for trending and analysis. (See Trending Data.)
• Monitor Alarms. (See Configuring and Processing Alarms.)
• Control equipment and processes. (See Defining Commands and Controls.)
• Store data in memory (See Configuring Local Variables.)

The most common variables supported by I/O Devices are digital and integer variables, although some I/O Devices support other numeric variables and strings.

See Also