ControlLogix support for adding I/O online

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Rockwell Allen-Bradley's ControlLogixA common client question of late is what are the abilities of the ControlLogix when is comes to adding I/O online.

In certain industries, processes can’t shut down as doing so would lead to loss of production or in extreme cases damage to the production line. In these situations, the more you can do while the production line is running, the better off you are. Allen-Bradley has had robust online program editing for decades, as well as I/O support for RIUP (removal and insertion under power,) but the online addition of I/O in the ControlLogix family is a more recent development.

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The first phase of support for this feature was in version 15 of the ControlLogix. In that version you could add most 1756 I/O while the processor was in the remote run mode, either in the local 1756 I/O rack with the processor, or remotely on ControlNet or Ethernet.

When adding I/O online on ControlNet, the I/O is placed in the unscheduled bandwidth portion of the “Network Update Time” (aka NUT) which would be more than adequate for most process applications, by may not have the update rate and repeatability needed for high speed applications.

Rockwell Allen-Bradley's Logix-WP006 Table 1
Table 1 from Allen-Bradley Publication Logix-WP006

Note: For details about v15’s support for adding 1756 online in remote run, please reference the following manual from Allen-Bradley (A-B:)

Runtime Addition of I/O White Paper, LOGIX-WP006

The second phase of support for adding I/O online with ControlLogix came with version 18. This version added many new devices to the support list, including:

  • 1756-CN2, 1756-CN2R, 1756-CN2RXT
  •  1756-EN2t, 1756-EN2TR, 1756-EN2TRXT, 1756-EN3TR
  • PowerFlex 4, 40, 400, 400P
  • PowerFlex 70, 700, 700s, 7000
  • PowerFlex 753, 755
  • PowerFlex DC
  • 1305, 1336E, 1336F, 1336R, 1336S, 1336T, 1397
  • 1757- FFLD

Note: For details about v18’s added support for adding devices to a ControlLogix system online in remote run, please reference the following manual from Allen-Bradley (A-B:)

ControlLogix System User Manual 1756-UM001

I hope you’ve found the above information and links about adding I/O online to a ControlLogix helpful. If you have a question or comment please use the “Leave a reply” form below to comment on this article.


Shawn Tierney
Automation Blogger and Trainer

Need personalized help? For as little as $25 per month you can get direct access to ask me Rockwell PLC, HMI, and SCADA questions! Learn the details at

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