# What is Class 3 communication in a Logix 5000 system?

**URL:** <https://forumautomation.com/t/what-is-class-3-communication-in-a-logix-5000-system/13390>\
**Category:** Electrical\
**Tags:** automation\
**Created:** [July 24, 2025, 9:41am UTC](https://forumautomation.com/t/what-is-class-3-communication-in-a-logix-5000-system/13390 "2025-07-24T09:41:23Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Rabert](https://forumautomation.com/user_avatar/forumautomation.com/rabert/32/6784_2.png) [@Rabert](https://forumautomation.com/u/Rabert)\
**Post date:** [July 24, 2025, 9:41am UTC](https://forumautomation.com/t/what-is-class-3-communication-in-a-logix-5000-system/13390/1 "2025-07-24T09:41:23Z")

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When a user program (or) HMI sends data on demand, it uses a class 3 connection.

 ![image](https://cdn.forumautomation.com/original/2X/8/8b5313ff0055ef8cbab7af7d9c1237053817aac7.png)

Class 3 connections are commonly utilized in Logix 5000 systems to transmit massive amounts of low-priority data.

Class 3 communications include

1. Logix CIP Data Table MSG instructions &

2. FTLinx Data Read operations.

Class 3 connections are opened & closed as needed.

A Class 3 connection can be ended manually by the originator or automatically after a specified timeout interval.

Opening a Class 3 connection makes use of a disconnected communication resource in the PLC.

If no resources are accessible, the consumer will receive an error message from the connection originator.

In current Logix 5000 systems, Class 3 communication is handled by a separate Communications core in the controller processor.

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