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How to Decode an I/O Module's Part Number

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As a PLC programmer, you need to know the functions and features of each I/O module in a system. You can quickly learn this information from an I/O module's part number.

In this edition of Learn Logix, we will see how to decode an I/O module's part number to determine the characteristics of the I/O module.

Decoding a Module's Part Number

An I/O module's part number gives us a lot of information about an I/O module, including it's product line, type, and characteristics.

Let's look at an example of a 1756-OB16D I/O module.

Part Number

The prefix of the part number identifies the product line that the I/O module is compatible with. Here, the prefix "1756" tells us that this I/O module is part of the ControlLogix product line. Other prefixes include 1769 for the CompactLogix product line, and the 5069 for the Compact I/O product line.

Product Line

The suffix of the part number identifies characteristics about the I/O module.

The first letter in the suffix indicates the direction of the I/O module. In our example, the "O" indicates that this is an output module. An "I" character is used for input modules.


The next character in the suffix indicates the type the of I/O module. In this example, "B" indicates that the module is a digital I/O module for DC power. Other types are "A" for digital I/O modules for AC power, and "F" for analog I/O modules.


The next characters in the suffix indicate the number of I/O points supported by the module. Here, the 16 indicates that this module has 16 individual I/O points. Typically, 1756 I/O modules include 8, 16, or 32 I/O points.


The final letter in the part number indicates the extra features that are supported by the module. "D" indicates that the module provides additional diagnostic capabilities like open wire detection. Other common suffixes include "E" to indicate the a module supports electronic fusing and "I" to indicate that each I/O point is isolated on a module.

Wrap Up

In this tutorial, we learned how to decode an I/O modules' part number to determine the module's;

  • The product line
  • Direction
  • Type
  • Number of points
  • Extra features

Now that you know this information, you can focus more on programming your system and less on looking up documentation for I/O modules.

When you're ready, I recommend that you learn more about PLC programming by following an affordable course based on free software.

PLC Programming From Scratch teaches you everything you need to know to be an intermediate PLC programmer by developing and testing the software for two industrial automation applications. By the end of the course, you will have two portfolio projects to discuss in interviews and you will know how to approach projects to design good PLC software.

Introduction to PLC Programming with CODESYS teaches you the fundamentals of PLC programming with CODESYS, a hardware independent PLC software development system that is becoming increasingly popular in industry.

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