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PLC Learning Series – Programming Steps

Developing a programmable logic controller (PLC) program can be broken down into five different steps. These programming steps are as follows:
Five Steps to PLC Program Development
Step 1 – Define the task
Step 2 – Define the Inputs and Outputs
Step 3 – Develop a logical sequence of operation
Step 4 – Develop the PLC program
Step 5 – Test the program
These five steps to PLC program development will help you in understanding, programming, and troubleshooting your automated machine.
PLC Learning Series – Programming StepsWe will be looking at each of these steps in a little more detail as we discuss the PLC programming development. Let’s get started.

Previously in this PLC Learning Series:
PLC Training Series – Tutorial for EveryoneVideo
What are PLC Inputs?Video
What are PLC Outputs?Video
Memory BackupVideo
Understanding NumbersVideo
Program Cyclic ScanVideo

A PLC programmer must know everything about the machine before they can program it. Five steps will be done for every PLC program written. Each step must be fully understood before continuing to the next. Sometimes during the next step, the previous one may be visited again. This is due to additional or clarifying information. This process will get you to a PLC program that will function as expected in operation. Watch the video below for an explanation of the five steps to PLC program development.
Here are some recent posts that discuss the five steps to PLC program development using examples.
Click PLC – Easy Transfer Line Programming – Video
Productivity PLC Simulator – Chain Conveyor MSVideo
See links below for additional program examples.

Step 1 – Define the task

What has to happen? This step is a collection of information about how the machine is to function. Conversations with owners, engineers, maintenance, operators, etc. are a few people to help you understand how the machine will function. I find open-ended questions are best to ask.
How do you picture this to operate?
What happens when…?
Do your similar existing machines have some current problems?
How is the machine maintained? (Mechanical Cycles, Oil Changes, etc)
Press Application
Write this information down and summarize your findings. The next step will sometimes force you to go back to individuals and ask further questions.  This is one of the most important steps for your program development. Good note-taking is important.
ASK Questions

Step 2 – Define the Inputs and Outputs

The information in step 1 will help you to determine the inputs and outputs required to perform the operations. This input and output list may also be provided to you. Review the requirements and ensure that everything is included.
The inputs and outputs refer to both discrete (on/off) and analog. Any special communications can also be included in this step. An example would be a temperature controller that will communicate Modbus RTU.

Master Switch – On/Off
Upper Limit Switch – On/Off
Lower Limit Switch – On/Off

Down Solenoid – On/Off
Up Solenoid – On/Off

If a control panel is required, make sure the inputs like start, stop, reset, emergency stop, etc are included. Outputs like lights, counters, etc are sometimes forgotten on the panel.

Step 3 – Develop a logical sequence of operation

This is where the majority of time is spent in PLC program development. Steps 1 and 2 allow you to systematically express what has to happen in the PLC program. Based on the logic, you may have to modify the inputs/outputs or sequence of the program. This is the easiest place to make changes.
PLC Learning Series – Programming Steps

This can be done with the use of a flow chart or sequence table. You can use anything to fully understand the logic of the operation before programming. Many people do not use this step and jump straight to programming.
PLC Learning Series – Programming StepsFully understanding the logic before starting to program can save you time and frustration.

Step 4 – Develop the PLC program

Utilizing the above steps, we will now actually write the plc program. This can be in several different languages. In the majority of cases, ladder logic is used.
Ladder Logic Program Code

Look at the sequence table with respect to the following logic. I have used Set and Reset conditions so it is easily followed by the sequence table. When the master switch turns on the up solenoid is activated. Notice the first rung is a direct correlation. Follow the rest of the sequence table with this ladder logic.
Document, Document, Document This is a vital part of every program, which will save you time and money when you have to return to the program years later.

Step 5 – Test the program

Test the logic that you have developed. Once again the previous steps are helpful in this process. First, start with what is referred to as ringing out the IO. This is where you would trigger the inputs and set the outputs to verify the wiring and communication. PLC program development testing is an important step to test for all conditions of the logic. (Power Cycle, Sensors Fail, Safety, etc.)
Press ApplicationTest the program with a simulator or actual machine. Make modifications as necessary. Check with the people most knowledgeable on the machine, to see if it is doing what they expect. Do they need anything else? Follow up after a time frame to see if any problems arise that need to be addressed.
PLC Learning Series – Programming StepsWatch the video below for a review of the 5 steps to PLC program development.

Additional examples of PLC program development using the 5 steps.
Click PLC – Easy Transfer Line Programming – Video
Productivity PLC Simulator – Chain Conveyor MSVideo
Five Steps to PLC Program Development – Die Stamping
PLC Programming Example – Process Mixer
PLC Programming Example – Shift Register (Conveyor Reject)
PLC Programming Example – Paint Spraying
PLC Programming Example – Delay Starting of 7 Motors
PLC Programming Example – Pick and Place
PLC Programming Example – Sorting Station (Shift Register)
PLC Programming Example – Palletizer

PLC Beginner’s Guide to PLC Programming
There are many different PLC manufacturers with different hardware and software. All of the programmable logic controllers have similar basic features. Here is how I would approach learning about basic PLCs.
Once you are familiar with the basics of the PLC you would then learn specifics for the controller that you will be programming.
This is the easiest way to learn about PLC programming.

Watch on YouTube: PLC Learning Series – Programming Steps

PLC Learning Series – Programming...
PLC Learning Series – Programming Steps

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