How To Troubleshoot With A PLC




How to Diagnosis and Correct Problems with RSLogix 500 and RSLogix 5000
2017 Edition
by Neal Babcock
ebook price: $19.95
Adobe PDF Edition

Troubleshooting is how engineers and technicians find and correct problems in a mechanical or electronic system.

It is a necessary skill for all those who are responsible for building or maintaining industrial systems and equipment. It is a part of their daily lives.

“How To Troubleshoot With A PLC” teaches you how to troubleshoot a system that is controlled by a PLC. It shows you how to interpret ladder logic to quickly find the problem with a machine.

There are two aspects of troubleshooting; one that is obvious and one that is not so obvious.

The obvious part is the technical understanding of the components of the PLC system and the software.

If you are working in industry now, you probably have a reasonably good understanding of the systems in your plant. You may, however, not have a good understanding of how to use the PLC software to trace and correct the faults in your systems.

“How To Troubleshoot With A PLC” uses examples written in Rockwell Automation’s RSLogix 500 and RSLogix 5000 (this software is used on Allen-Bradley SLC processors, and Allen-Bradley ControlLogix and CompactLogix processors). The book shows you, step by step, how to find the problem that is keeping your system from running.

The “not so obvious” part of troubleshooting is knowing the method. An experienced troubleshooter always adheres to a method. He understands the value of the method and knows that following the method provides the fastest results.

If you to want to take the quick path to learning PLC troubleshooting, this ebook is for you.

And, you can start today.

The book teaches you the method in a way that is easy to understand.

If you are an engineer or technician, and you work around equipment that is controlled by a PLC system, you must:

 be able to quickly diagnose problems using PLC software

 know how to connect to a processor that is online

 make minor changes in the program to get the machine running

 know how to test new ideas and new components

If you work with an Allen-Bradley programmable controller, you need to know how to diagnose and fix machines controlled with RSLogix 500 or RSLogix 5000. The book uses actual RSLogix to show you how to troubleshoot. This is an extremely important part of any automation training.

There are some basics that every PLC troubleshooter needs to know.

With “How To Troubleshoot With A PLC”:

 You will learn how to SAFELY make changes to a system without overriding machine safety or personnel safety programming.

 You will understand the value of backing up the program, so that the machine can be restored to “as it was” condition.

 You learn how to make temporary changes to a program to test new components or ideas.

 You have to know how to QUICKLY get an idea of where the problems exists.

 Provides you with more than enough “PLC basics” to get you going.

“How To Troubleshoot With A PLC” removes the barriers that keep you from going online with a PLC system and correcting the problem.

This book provides the necessary knowledge and automation training to help you get the PLC job you want.

Here are some excerpts from the book:


Automated Drill Press

The best way to learn how to troubleshoot is to look at a real world example with an automation controller.

Let’s say you get a phone call telling you that a machine is stopped. It seems to stop intermittently for no apparent reason. This machine is part of a production line; if it is not running, the whole line is stopped.

However, before you can do any troubleshooting, you must have a clear understanding of how the machine is supposed to work. This furniture manufacturer has a machine that drills a 3/8” hole in a certain spot on a piece of wood. The entire process is automated.

The main conveyor transports the part into the machine where the part meets a pneumatically actuated stop gate. At that time, another pneumatic cylinder actuates a clamp that pushes the part back against the conveyor wall. This holds the part in place during the drilling process. Photocells verify that the part is in position; the spindle lowers and proceeds to drill a hole in the part. After the hole has been drilled and the spindle has retracted to its home position, the clamp releases, the stop gate raises and the part exits.

Troubleshooting the Logic
As we go through these rungs, make sure that you understand how each rung works. If you do not understand the purpose of the individual rungs, you will not be able to troubleshoot the system.

We are now online with the processor, so let’s take a look at the first rung.



We should first look at the output and notice that it is not true, or off.

The instructions that are highlighted in green are true. In order for the OTE instruction (System Running B3:0/0) to be true, the must be a path of logic leading to the output that must be true.

Written in pure Boolean form, we can say that:

If the emergency stop is cleared and the guard is in place and there is not a system fault and the start button is pressed (or if the start button is not pressed, the system is running) and the stop button is not pressed the system running bit is on.

To make sure we understand, let’s look at each instruction in detail.

EmergStop Cleared CR1 I:1/4
This is a hardwired input, as indicated by the “I” in the address. It comes from relay CR1 (CR is an electrical abbreviation for Control Relay).

E-Stop pushbuttons are wired so that when the button is pushed, an emergency stop is initiated. You will see these at any gas station. It is much faster to push a button than to pull it; the operator can just “hit” the e-stop to stop the system. This is, obviously, the quickest way to stop a system in an emergency.

Most e-stop pushbuttons are 2-position, maintained switches. You can pull on the switch to place it in the first position, or push it in to place it in the second position. It will “maintain” the position in which it was last placed.

Sometimes e-stop switches are used in conjunction with a relay, so that other switches or sensors may be placed in series with it. These components are then wired to the coil of a relay. Wiring the circuit in this way provides the most fail-safe method of assuring that the e-stop circuit will work when it needs to.

We may not have electrical prints for the machine, but we can assume that if the E-Stop pushbutton is pulled out, it is “cleared” and will allow the machine to run. The e-stop instruction is highlighted with green, so we see that the instruction is true, and it is not keeping the machine from running.

Guard in Place LS26 I:2/4
Most industrial machinery has a guard to keep people from getting hurt by being inside the machine when it is running. A guard can be anything from a personnel gate to a Plexiglas window. In our case, it is a Plexiglas shield that swings down in front of the spindle.

LS26 is a limit switch that is hardwired into the PLC as an input. It is highlighted green, so that is not our problem

System Fault B3:0/12
The “B” in the address tells us that this is an internal bit. Also, because the instruction is normally closed and it is false, we know that bit B3:0/12 is on. Something is turning on the System Fault bit, somewhere, and it is keeping us from running. This is where we will begin tracing the logic, but first let’s complete our understanding of the rung.

Start System PB2 I:1/1
This is a hardwired input coming from the “Start” pushbutton. Typically, a start button is a momentary switch; it only makes contact if it is being pushed. That is why there is a branch around it. With everything else in the rung being true, when the button is pushed, the “System Running” bit B3:0/0 will come on. The XIC instruction in the branch will be true and the “Start” button can be released. The system will continue to run.

Stop System PB3 I:1/2
This pushbutton is wired to a hardwired input in a fail-safe manner. That is why the instruction is an XIC, or normally open. If the wire to the input is removed, or cut, the system would not run.

You probably will troubleshoot systems that have rungs that are much more complex than this rung. As you become more adept at troubleshooting, you will be able to quickly scan a rung from left to right and look for bits that are not true (not highlighted). In this case, the “System Fault” bit really jumps out.
RSLogix 500 allows a user to customize colors. Green is the default color to indicate an instruction is true. However, you may, at some point, pick up someone else’s laptop and the colors might be modified.

We can clearly see that the emergency stop button, the machine guard or the “Stop” button instructions are not keeping the machine from running. No one is pressing the “Start” button, so we would not expect that to be true.

The culprit is the “System Fault” bit (B3:0/12).

The “B” in the “System Fault” address tells us that it is binary bit, also known as an “internal” bit. Let’s track down this bit.

Searching the Program
Right-click on the instruction and choose “Find All”.



The search results window appears.

It tells us that the bit B3:0/12 appears in File 2 on Rung 33 as an OTE, or output.

It also tells us that the bit appears in Rung 0, which is the rung we are already viewing (searches in RSLogix return all occurrences of the bit).

Click on the first line of the search results window and Rung 33 appears.


First, take note of the fact the output instruction is true. This explains why the XIO, or normally closed, System Fault instruction in Rung 0 is false.



Modifying A Program To Trap Faults

Let’s add some logic to latch these faults. Close the search results window and double-click on the rung number for Rung 19. You will see that a copy of the rung is created for you to edit. The original rung is still shown below. Until you edit the copy and accept it, the program will continue to run the logic in the original rung.


Right-click on the branch and select “Add Branch Level”.



The screen looks like this.

Click on the “Examine On” button in the tool bar and drag it to the new branch you have created.


Double-click on the tag name field (currently occupied by the question mark) above the instruction. Use the drop-down menu to navigate to the tag “ValveAVQRFault.DN”.


The Overall Method

Before we learn how to troubleshoot a PLC, we first must understand the basics of troubleshooting.

That really isn’t as hard as it might seem; the basics of troubleshooting revolve simply around creating a clear goal and developing a concise plan.

The book thoroughly explains “The 6 Steps to Creating a Clear Goal” and “The 6 Steps to Creating a Clear Plan”.

After you understand that, we take you through real-world examples of troubleshooting machines and systems.

You will get 102 pages of text and images showing you how to troubleshoot RSLogix 500 and RSLogix 5000. The total package includes:

• How To Troubleshoot With A PLC.pdf

• Drill Press – RSLogix 500.pdf
A printout of the RSLogix 500 drill press program

• Batching – RSLogix 5000.pdf
A printout of the RSLogix 5000 Batching program

• Batching – RSLogix 500.pdf
A printout of the RSLogix 500 Batching program

This ebook is the result of many years of experience in PLC troubleshooting.


“This book has the shortcuts I needed to learn.”

Harry Levitt

“It is easy to read, easy to understand and well worth the money for anyone who deals with a programmable controller.”

Marion Van Amburg
Project Engineer

“I have been troubleshooting equipment for years. This brought me up to speed with PLC troubleshooting.”

Vance Cummings
Instrument Tech

About the author
Neal Babcock is an Electrical Engineer and has written and maintained dozens of PLC programs used in the semiconductor, automotive, energy and aviation industries.

Our Returns Policy
Within 14 days of receipt of your download, you may return any of the items you purchased from Engineer and Technician, for any reason, for a full refund.

“How To Troubleshoot With A PLC” is the fastest and easiest way to learn PLC troubleshooting. Available for instant download.

Product details
Format: Adobe PDF (ebook),
File Size: 4114 KB
Print Length: 109 pages
Publisher: Modern Automation, LLC
Publication Date: January 7, 2017
Language: English
Text-to-Speech: Enabled, with user software
Lending: Not Enabled
Screen Reader: Adobe Reader, Apple Preview, others