Tag Archives: plc troubleshooting example

Productivity 1000 Series PLC Numbering Systems and Tag Database

The Productivity Suite Software allows us to use tags in the PLC. Tags are a method for assigning and referencing memory locations within the programmable logic controller. They allow a more structured programming approach and are stored within a tag database. The tag database is stored in the memory of the Productivity Series of PLC’s from Automation Direct. Do not over think tags. Tags are just names that we assign to variables of any data type stored in the PLC memory.

We will be looking at data types available in the Productivity PLC and how to use the tag database. Let’s get started. Continue Reading!

Productivity 1000 Series PLC Monitoring and Testing the Program

The Productivity Suite Software provides tools for us to monitor and test our programs. Last time we used Tag Names and Details, Task Names and Descriptions, Rung Comments and Instruction comments to document our first program and transferred this to our connected Productivity 1000 Series PLC. We will be monitoring our ladder using the ladder editor window and display the information two different ways. Data View is a powerful tool to help us to test and view our program. We will be forcing the IO, toggling the IO view and graphing our tags to test our PLC logic circuit.
Let’s get started. Continue Reading!

BRX PLC Online Editing and Debug Mode

Our first program for the BRX PLC involved a start stop jog circuit. We will now use the Do-More Designer Software to modify this program while the PLC is scanning the logic. This is referred to as online programming or online editing. As a system integrator, this ability can prove very useful to you in the field when commissioning your automation system.
We will also be using the special Debug Mode of the software to monitor the ladder logic status and bits on a scan by scan basis. Forcing the inputs and outputs, we will control the execution of the PLC scan. This is a great feature of the Do-More Designer software.
Here is a link explaining the logic behind our circuit.
How to Make a Start Stop Jog Circuit in a PLC
Let’s get started. Continue Reading!

How to Troubleshoot a PLC

Your control system does not work. Where do you start? Let’s walk through a series of questions in order to determine where the problem lies.


Is this a new installation or previous installation that was running fine? Determine if the system has been running well in the past and has currently stopped working correctly. This is the indication that the problem relies upon the system.

Is there anything that has happened outside of the system? Has there been a lightning strike, blown drives on other systems, etc?  This can point to the original cause of the malfunction.


What is the system doing now and what should it be doing? Gather all of the information you can from every resource you can.

  • Supervisors  – machine, location, time of the error, other happenings in the plant, etc
  • Operators – What is it currently doing? What should it be doing? What do you think is wrong?
Operators of the equipment are your key resource in finding, correcting and ensuring the error does not happen again. They know the equipment from an operational point of view which can assist you greatly in troubleshooting.

PLC fatal and non-fatal errors:
If the machine is still running partially then this is an indication of a non-fatal error. Cannot run at all is usually a fatal error.

Do_More CPU Units

Take a look at the PLC indicator lights on the CPU. Refer to the operation manual for the PLC for troubleshooting specific lights on the CPU. The following are general tips:

If no lights are on then the possible cause is a power supply. This is usually the most common of errors on a PLC system. Mean time before failure (MTBF) is rated on the lowest rating of components which is usually the power supply.

If the run light is on and an error light flashing this usually indicates internal errors such as batteries, scan time, etc. It is usually not the reason for the lack of operation.

If the run light is on and no other errors are seen on the CPU we can put the PLC program on the bottom of the list of items that could be the cause.

Check the input cards of the PLC. You should see the individual sensors lighting up the inputs. If not then check the power supply to the input card/cards.

Ask the operator what is happening and what is suppose to happen. Try to follow the sequence of events in the PLC to determine either an input or output device not working.
Some items to watch:

If this is a new PLC program that you are doing start with a logic flow diagram. This will determine the procedure to start programming.
Every program can be done in several ways. The best method is the most documented one.

Documentation is the mark of a good program.


Some trouble with new programs can be racing conditions. This is usually a case of not understanding how the PLC scans logic. In general, the PLC will scan from left to right, top to bottom. The output bits/words are available to the inputs of the next rung of logic. (Modicon PLC’s will scan differently.) Actual outputs and inputs are not read until the end of the scan of the PLC. Racing conditions happen when the output is set on multiple rungs, but will not get actually set until the end of the scan. Think of it as the last action will always win. So if this happens to move the logic to the end of the program and see if it works. Then go back and see where the output was also set.  Cross-reference guides are ideal for this purpose. (Refer to your programming software on how to get cross-references.)

start stop 011

We have discussed just a few troubleshooting techniques. Hopefully, now you know how to start looking for the errors on your system. Let me know how you make out.

Watch on YouTube: How to Troubleshoot a PLC

Do you know of additional tips or methods to share?

If you have any questions or need further information please contact me.
Thank you,

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