How Do You Test a PLC Program?

You can use various methods to test a PLC program, including emulation, simulation and in-the-loop real performance.

• Simulation: when you run the program in a virtual environment, using simulation software.

• Emulation: when you run the program on another computer to see how it performs.

• In-the-loop: when you run the program on actual PLC hardware, and see how it performs with the devices and processes that you want it to control.

Step-By-Step Testing For a PLC Program

1. Objectives

Before you start testing the program, you’ll need to determine what your objectives are:

• What do you expect to achieve by running this test? 

• What are the criteria for success and the expected outcomes? 

• How will you assess the results and document them? 

How you answer will determine how you design your test scenarios and plan out what procedures you want to analyse. It will also help you identify any potential challenges or risks that may emerge during the test, so that you can prepare a backup plan.

2. Choosing The Test Environment

You might find that you need several levels of test environment, depending on how complex your PLC program is and its anticipated scope. 

Simulation software tools offer probably the easiest and most accessible means of testing your PLC program. You can recreate your system’s real-world I/Os in a virtual environment and monitor the results. No hardware or wiring is required to optimise performance, verify functionality, identify errors and fix them. 

Emulation mode allows you to execute your PLC program on an ordinary computer, emulating the PLC’s hardware configuration in order to test and debug it. You’ll find it more realistic and accurate than a simulation, since it uses the firmware and memory from the actual PLC. However, you’ll still need to provide your program with inputs and outputs (I/Os), which means connecting external devices like actuators, sensors and switches. Emulation benefits may be limited by inadequate memory, communication and speed, however, depending on what PLC software and computer you use.

You’ll save time and money with simulation, but it’s always possible that your PLC system may encounter conditions in the field not accounted for in the software. This might be the time to run your program in the real world, to put your hardware in the loop for some additional program testing and debugging. 

3. Debugging

You’ll find that most PLC programming software offers integral debugging features, so you can monitor and modify your program even as it’s still running. These include: 

• Watch windows displaying real-time values of selected variables.

• Breakpoints where you can pause the program to inspect the values of I/Os, registers and variables.

• Force tables where you can override such values.

• Traces showing your PLC program’s history and sequence of events. 

With these debugging tools, you’ll be able to simulate different scenarios, track changes and monitor data trends. 

4. Further Tests of Your PLC Program

Whichever testing method you choose, you’ll need to prove that the program is functional in a range of conditions, both normal and abnormal. You’ll be testing its operations, performance and logic, using guides such as checklists, scripts and test cases. Automated testing tools are also available.

After this, you should ensure that your PLC program is communicating correctly and is properly integrated with other systems and devices such as HMIs, actuators, sensors or even other PLCs. This means testing their protocols, interfaces and exchange of data, using tools like communication simulators or protocol and network analysers. Validation testing is a final step that proves your PLC program is fit for purpose. This will demand real, or almost real-world, testing of the program’s navigability, reliability, and safety, via user surveys, interviews and feedback. 

Testing Your PLC Program

Emulation is probably the most realistic of the available options, as you’ll be executing the program using real PLC hardware, even if you simulate the control processes. Simulation is a cheaper and simpler option, which can mimic what actions you require the program to perform in virtual reality. Running your program on real PLC hardware that’s connected to actual functioning devices will give you the most accurate test results. It can be risky if it turns out that there’s any glitch in the program, but you’ll know for sure what it can and can’t do.