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LJM User's Guide

Welcome to the LJM User's Guide! This document describes the API and usage of the LabJack LJM library.  The LabJack LJM library is a cross platform library that allows programs to read and write Modbus registers implemented on a variety of LabJack devices.  A searchable list of all of the supported registers that can be read from or written to is available on our Modbus Map page.

Supported Devices

LJM supports T-series devices:

LJM also supports the Digit, which is deprecated.

Software support for other devices can be found on the Software page.

LJM Download

See here for the LJM Installer.


Example code for LJM is available in Python, LabVIEW, C/C++, and other languages.


Reading This Document

Table of Contents Note

Navigating using the Table of Contents Table of Contents button on mobile

An efficient way to navigate this online document is to use the Table of Contents button to the left.

Offline PDF Note

Offline PDF

If you are looking at a PDF, hardcopy, or other downloaded offline version of this document, realize that it is possibly out-of-date since the original is an online document.  Also, this document is designed as online documentation, so the formatting of an offline version might be less than perfect.

To make a PDF of this entire document including all child pages, click "Save as PDF" towards the bottom-right of this page.  Doing so converts these pages to a PDF on-the-fly, using the latest content, and can take 20-30 seconds.  Make sure you have a current browser (we mostly test in Firefox and Chrome) and the current version of Acrobat Reader.  If it is not working for you, rather than a normal click of "Save as PDF" do a right-click and select "Save link as" or similar.  Then wait 20-30 seconds and a dialog box will pop up asking you where to save the PDF.  Then you can open it in the real Acrobat Reader rather than embedded in a browser.

Rather than downloading, though, we encourage you to use this web-based documentation.  Some advantages:

  • We can quickly improve and update content.
  • Click-able links to further or related details throughout the online document.
  • The site search includes this document, the forum, and all other resources at labjack.com.  When you are looking for something try using the site search.
  • For support, try going to the applicable page and post a comment.  When appropriate we can then immediately add/change content on that page to address the question.

Periodically we use the "Save as PDF" feature to export a PDF and attach it to this page (below).


A PDF (or some downloadable form) of the LJM Library User's Guide would be super useful!



I've added a compiled .PDF of the entire LJM User's Guide to the File Attachments section of this page! Happy coding!

Or, try the "Export All" link on this page! It will compile a .PDF of the entire LJM User's Guide then start a download.

Best regards!

Edit: Added precompiled .PDF attachment