I believe that in the world of control systems, the little things can make a huge impact.

The little things we do ahead of problems will determine how well we diagnose and repair issues when they arise. It can mean the difference in troubleshooting time in seconds instead of hours.

Image by: Brandon Cooper

Sometimes, the little things are available, but not utilized. Especially in a PLC system, the functions are available, but they must be implemented.

For instance, if you are using diagnostic I/O modules in a ControlLogix system, there are parameters available for monitoring and interacting with the modules, but many times, these parameters go without utilization.

Maybe today is the day to change that. Maybe today, we change the details of our systems to aid the troubleshooter to diagnose and repair failure as efficiently as possible.

Diagnostic Information

So, what information is available? Let’s take-a-look at for instance a 1756-OA8D module. There are several monitoring parameters available:


This parameter will indicate that an electronic fuse inside the module has tripped. This is not a real fuse. It can be reset inside the module properties in Studio 5000 or we will later see, in this article, that it can be reset in logic.


This parameter detects no leakage current through the output load, most likely indicating a broken circuit


This parameter means that the Output Module was unable to detect any change of state at the output circuitry even though a change from “OFF” to “ON” was ordered.


This parameter detects that no 120 VAC Power is present on the incoming side of the module relay. Look at drawings to determine where external power comes from.

The following indications would be the parameters that could be monitored or alarmed for one channel of one I/O Module. What can be done with this is simply a limit of the imagination.

Image by: Brandon Cooper

Using the PLC Program to Interact with the Module

I mentioned above that the module’s electronic fuse can be reset using ladder logic and we will take-a-look here at how that can be accomplished. In the screenshot below, if a fuse is blown, an external reset button could trigger an MSG instruction to write to the I/O Module.

Image by: Brandon Cooper

Inside the MSG instruction, the tabs will be set up as follows:

Image by: Brandon Cooper

The communication path is the path to the I/O Module in the Studio 5000 I/O Configuration Tree:

Image by: Brandon Cooper


One of my main personal drivers is to make something better every day. What we do with the little things can make someone else’s life easier as well as mitigate or reduce downtime and that is what it is all about.

If I, as a control system engineer, can make just one improvement a day, think about what a year can bring.

Written by Brandon Cooper
Senior Controls Engineer and Freelance Writer

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Brandon Cooper



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