# About data sources

The FactoryTalk View documentation uses the term data source as a generic term that includes all possible sources of tag data, for both data server tags and HMI tags.

The data source can be memory or a device such as a programmable logic controller or OPC server. The application writes values to and reads values from the data source.

The data source is configured to exchange information (in the form of numeric or string values) between the application and the physical machine that your project is controlling.

The data source for analog, digital, or string HMI tags can be Device or Memory. For system tags, the data source is Memory.

## Device

When you choose Device as the data source for a tag, you have to specify where the value for the tag is obtained. This can be a memory location in a programmable controller or an OPC Item.

## Memory

The data for a tag with Memory as its data source comes only from the value table. A memory tag is not associated with a programmable controller. Its value at startup is defined by the Initial Value setting. The current value can be changed using an object in a graphic display.

## System

When you create a new project, the application creates system tags. They are in a folder called System and have System as their data source. They store information that is generated while the system is running, such as the current system time and date. You cannot create or edit system tags, but you can use them wherever you would use any other tag.

Important: | Do not create HMI tags that are pointing to a System tag. When a project contains an HMI tag pointing to a System tag, users may fail in creating an MER or have runtime performance issues.

All tag values, whatever their source, are stored in the value table. All parts of the project, including the graphic displays, alarms, and so on, read the values from this table.

## See also

About the Device data source

About the Memory data source

About OLE for Process Control (OPC)

About HMI tag types

value table

Keywords: data source