CompactLogix Modbus FiWhile Ethernet/IP leads the way for new industrial network installations in North America, there’s no denying the massive number of products that still ship with some sort of Modbus protocol support.

In fact, Rockwell’s own “DSI” protocol used in the PowerFlex family of drives is Modbus RTU compatible. And as we’ve previously discussed, most of the MicroLogix line supports Modbus in one form or another.

That said, the CompactLogix line is currently a very popular choice for Programmable Automation Controllers (PAC,) and in today’s article we’ll discuss which models in this family support Modbus.

CompactLogix support for Modbus RTU

1769-L35ECompactLogix models with a built-in DB9 serial port don’t natively support Modbus RTU like most MicroLogix do.

However, the folks at Rockwell Automation have been providing sample code to use the DB9 serial port as a Modbus RTU Master or Slave since version 13 of RSLogix 5000.

That sample code is installed automatically with the rest of the RSLogix 5000 samples, but it can also be found on Rockwell’s sample code website,

Having used the code myself, I can say it’s not for the faint at heart. But, if you need your CompactLogix’s serial port to communicate using Modbus RTU, you should check out this free sample code from Rockwell to see if it’l meet your needs.

What if my CompactLogix doesn’t have a serial port? Or if this sample code is too limited for my application?

Prosoft-1769-ModulesIf you’re using one of the new 5370 CompactLogix processors, you probably noticed you have a nifty new USB port in place of the legacy DB9 serial port.

So without a built-in DB9, once choice for adding Modbus RTU capabilities to your ComapctLogix is to use an expansion I/O communications module.

However, even with a ComapctLogix that has a DB9, you may still opt for one of these add-on modules if the sample code mentioned above doesn’t support the amount of data your application requires.

Rockwell’s encompass partner, Prosoft Technologies, makes several Modbus expansion I/O modules for the CompactLogix family, and I’ll list the most common ones below:

  • MVI69L-MBS, Modbus Serial Lite 1769 Communication Module for CompactLogix
  • MVI69E-MBS, Modbus Serial Enhanced 1769 Communication Module
  • MVI69-MCM, Modbus Master/Slave Network 1769 Interface Module for CompactLogix
  • ILX34-MBS485, Modbus RS485 Serial Module for CompactLogix L1 and Point I/O Adapters
  • ILX34-MBS232, Modbus RS232 Serial Module for CompactLogix L1 and Point I/O Adapters

CompactLogix support for Modbus TCP

1769-L36ERM FrontThe new CompactLogix 5370 models come with an updated Ethernet port which supports open sockets.

And on the Rockwell’s sample code website you’ll find a sample application which uses the open sockets feature to give these new CompactLogix models the ability to be either a master or slave on a Modbus TCP network.

Having used this code recently, I can say it’s well documented and fairly easy to use. That said, unfortunately it’s not quite as easy as using the MicroLogix 1400 on Modbus TCP.

(I really wish the wizards in the MicroLogix group would share their secretes of putting these protocols into firmware with the CompactLogix folks!)

1769-L32E-FrontWhat if I have an older CompactLogix?

If you’re not using one of the new 5370 CompactLogix processors, your Ethernet port doesn’t support open sockets, and therefore the above sample code isn’t going to work for you.

In that case, or in the case where the capabilities of the sample code is too limited for your 5370 application, we again have the option of adding Modbus TCP capabilities using an expansion I/O communications module.

Prosoft Technologies also makes Modbus TCP modules, including these versions for CompactLogix processors:

  • MVI69L-MBTCP, Modbus TCP/IP Lite Communication Module for CompactLogix
  • MVI69E-MBTCP, Modbus TCP/IP Enhanced Communication Module
  • MVI69-MNETC, Modbus TCP/IP Multi Client Network Interface Module for CompactLogix
  • MVI69-MNET, Modbus TCP/IP Communication Module for CompactLogix

I hope you’ve found the above information about which models of CompactLogix support Modbus helpful.

If you have any comments, questions, corrections or suggestions, please feel free to share them with us by submitting the “post a comment or question” link below.

Until next time, Peace ✌️ 

Shawn M Tierney
Technology Enthusiast & Content Creator

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  1. Shawn,

    I come across your blog a lot from google. Thank you for the information you provide.

    Is there any way to know if I am using a “new” or an “old” CompactLogix 5370? I have many of these. Some were purchased within the past few years, others within the past few months.

    • Sorry, I left out some more information. I’m looking for compatibility with raw sockets (for Modbus TCP functionality).

      It it tied to controller revisions or specific models? I’m using the 1769-L36ERM currently.

      Thanks again.


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