---
title: "Setting a Tag's Scan Class"
---

# Setting a Tag's Scan Class

> <IUCard title="Setting a Tag's Scan Class" link="https://www.inductiveuniversity.com/video/setting-a-tags-scan-class/7.9" /><br />

Each Tag in Ignition is assigned a scan class which dictates the polling rate and conditions on which the Tag will be evaluated. For example, the scan class will dictate how often that value is going to poll from the PLC if it's an OPC Tag, or how often the expression is going to run if it is an Expression Tag, or how often the value is going to query the database if it is a Query Tag.  Whatever type of Tag you're using, you can set a scan class on that Tag. You can also specify both a Realtime scan class and a Historical scan class for each Tag.

The three different [scan classes](tags\scan-classes\scan-classes.md) that specify the rate of execution of Tags are as follows:

- [Direct](direct-scan-class.md) executes based on the slow polling rate setting at a fixed rate.
- [Driven](driven-scan-class.md) executes based on a condition.
- [Leased](leased-scan-class.md) executes according to on-demand polling.

## To Set a Scan Class for a Tag

1. In the **Tag Browser**, right-click on any Tag, and click **Edit tag(s)**. 

    The **Tag Editor** window is displayed.
 
2. On the **General** tab, choose a scan class from the **Scan Class** drop-down for the selected Tag.

    For example, choose the **Default Historical** scan class.
 
3. Click **OK** to save it.

    You will immediately see the Tag updating at a different rate.

    ![" "](<setting-a-tags-scan-class/1-Editing a Tags Scan Class.png>)

4. The Tag also uses the scan class to determine how fast to log data for the Historian. The Historical Scan Class doesn't affect how fast the values get polled from the PLC, but affects how fast the data gets logged to a database. On the **History** tab, you can set the Historical Scan Class to **Default Historical**.

    ![" "](<setting-a-tags-scan-class/2-Editing a Tags History Scan Class.png>)

    As you can see, setting scan classes on a Tag is extremely easy. You can even select multiple Tags in the Tag Browser by right clicking to edit the Tags. This opens the Tag Editor, and simply set the scan class for all the selected Tags at the same time.  

    ![" "](<setting-a-tags-scan-class/3-Multiple Tags Selected.png>)