Siemens PLC Error LED: S7-300 CPU Status and L.E.D. Error Explained

If you are using the S7-300 CPU, it is pretty normal for you to experience an SF fault on Siemens PLC from time to time. People who use this device have occasionally complained about encountering one fault during usage.

Whenever you experience this problem, you should know that the problem is most likely from the System’s CPU. You might be confused about what to do when you experience an SF fault on Siemens PLC.

You do not have to get confused anymore. In this article, we will discuss everything from the SF fault on Siemens PLC. We will also discuss the error codes and what you can do when you experience an SF fault on Siemens PLC. 

What is SF Fault LED on Siemens PLC?

This is one question many people tend to ask when it comes to Siemens PLC and how to fix this SF fault Siemens PLC. If you notice that the SF fault LED light is turning on, it means there is likely a problem with Siemens PLC.

It could be a hardware problem while it can also be a software problem. It all depends on where the fault is coming from. If you notice that the light is coming on with the BFx led light, it simply means there is difficulty with the program trying to access data from one of the faulty remote components.

Now that you know what the SF fault on Siemens PLC is, we can discuss the different types of LED lights on Siemens PLC.

Types of LED. Light and What They Mean

There are different types of LED lights on your Siemens PLC, each of which has meaning and usefulness. Below is a list of the led lights and their meaning. You should know that these LED lights are very important as they tell you the current state or status of your Siemens PLC. 

Types of LED. Light, What They Mean


This is the first LED light you consider important when you see it turning on your Siemens PLC. If you see it turning on, it simply means you expect an SF fault on Siemens PLC. If the light turns on, it means there is a problem with the hardware. The only solution is for you to find the hardware solution and fix it in time.

The SF light can also turn on if there is a problem with the software programming as it turns on for software and hardware errors. If you have a software error, you must find ways to reprogram the PLC. The SF light is a steady red and stays on if there is a fault.


This is another thing that shows there is an error somewhere. While this is completely different from the SF fault on your Siemens PLC, it is still equally as important as the SF fault.

If you are getting a BF fault, retrieving information from one of its external sources is a problem. The light for the BF fault is a steady red, just like the light for the SF fault on Siemens PLC. The major difference, however, is that it is beneath the LED for the SF fault on Siemens PLC. 

If you get a BF fault, you should know a problem or an error in the system Network. It could be a bar contact or bad wiring in any communication connectors. It could also mean an overlap exists in the addresses of the network. It is relatively easy to fix this problem. You only need to take your time to discover where the communication problem is coming from and fix the wiring or connection.


While this indicator doesn’t come up too often, if ever it does come up, it means there is a problem somewhere. This MAINT led light is just as important as the SF fault on Siemens PLC led light. The colour of this light is a steady yellow, which means a problem with the CPU.

When it comes on and stays on, it usually means that the CPU in your PLC isn’t working anymore. You will have to take your time to remove and service the CPU if you want it to start working perfectly again.

The moment this is done, the MAINT light turns off by itself almost immediately.


This is another led light you should watch out for when looking at the faults on your Siemens PLC, even if it is SF faults on Siemens PLC. If the light comes on, your CPU is receiving a 24volts DC. You need to take your time to check the system requirements to know if such volts are within the voltage allowed for the CPU. Different PLC CPUs receive different volts. Check the one that is perfect for you in your user manual or instructions. The colour of the LED light is a steady green


FRCE is another led light that sometimes tells you when there is danger. This led light which is a steady yellow, allows you to know when one of them isn’t working.

When the steady yellow light comes on, it simply means that one of the input or output devices has been forced to stop. It could be that the force was triggered manually, while it could also be that the PLC did the force. However, once you see the light, one of the components has been forced to stop.


This is another LED light you should take note of. The good thing about this light, unlike other lights, is the fact that this light doesn’t mean there is something wrong with your PLC. On the contrary, it simply means everything is working perfectly with your System. When the run light, a steady green LED light, is on, it means there is nothing wrong with your PLC or your CPU as everything is working correctly and perfectly. You do not have to worry about anything when you see this light.


This LED light is a steady yellow. When it comes on, it means the CPU has been forced to a stop, and if you notice that the STOP-led light is staying on, you should know there is a problem. The problem, just like the SF fault on Siemens PLC led light, could be a hardware or a software problem. You will have to call the engineer to detect the exact problem if you are not too familiar with the PLC system. When it is fixed, the STOP LED light turns off as soon as the CPU works perfectly. You should note other LED lights when it comes to Siemens PLC. 

Below are the led lights;

Memory Reset Request

This usually happens when you slot a new memory card into the System. The LED light will flash yellow slowly, which means that your memory reset request has been received.

Memory Reset is being Carried out

Once the memory reset request has been received, the next thing that would happen is the reset being carried out.

If the reset is carried out, the yellow light will flash faster for some time. 

Now that we have discussed the LED lights when it comes to SF fault on Siemens PLC, you should next know the error codes on Siemens PLC and what they mean. The error codes will be discussed below.

Error Codes on Siemens PLC

Aside from encountering an SF fault on Siemens PLC, there are different error codes you should note if you are using Siemens PLC. These error codes are very important as they let you know when something is wrong with your device or components. Below is a list of some of the error codes you might find important;


Once you get this error message, it means there is a problem somewhere with your hardware. This is because this error message only appears when there is a hardware fault.

It could be that one of the components wasn’t properly installed. It could also be that there are one or two issues with some components that are not working properly. You must check your hardware to confirm where the fault is coming from.


This is another error code you might see from time to time on your PLC. When you see this error code, it simply means objects were not allowed.

This usually happens when the access to the timer and the counter data is not set to BSD.


When this error code occurs, it simply means you are trying to act as the PLC context. When you see this error code, you will see a beyond-context message.


This is one error code you should watch out for. When you see this error code, you are trying to request an address beyond the PLC range. It could also be that you are requesting an address that doesn’t exist on the data block.


Experiencing an SF fault on Siemens PLC can be new to you, but we tried to address the issue in this article, such as the causes. You will find different types of LED lights on your Siemens PLC, and some of the led lights could mean an SF fault on Siemens PLC. 

We have discussed these led lights, the things you can do to remedy them, and the error codes you might see when using Siemens PLC. All you need to do is read the article and know everything about the SF fault on Siemens PLC.


A Guide to Programmable Logic Controllers (PLCs): Types, Applications, and Basics

Programmable logic controllers (plc) are mini-computers that help to boost productivity across most sectors in the manufacturing industry. With its importance and relevance in the manufacturing industry, one question is ” what is a PLC controller.”  

The PLC controller is everywhere in most large-scale manufacturing or production industries ranging from oil and gas to mining, glass, textile, and much more. 

With its usefulness among large-scale industries, you might have a lot of questions when it comes to this mini-computer. If you have questions such as ” what is a plc controller? Plc vs computer, and a lot more, you do not have to worry.

We will be taking our time to answer every possible question you might have regarding what a PLC controller is and a lot more.

What is a PLC Controller?

When it comes to a plc controller, the questions many people tend to ask are ” what is a plc controller? What exactly do they do? How expensive are they? Plc vs computer, which is better?”. We are going to be answering all these questions in this article. Let’s start with the first question.

PLCs can be called mini-computer. This is because they perform the functions or tasks of a computer. They do it differently. PLCs are majorly used in the manufacturing processes of significant industries today.


what are plc controllers


If you want a simple answer to the question, what is a plc controller? The plc controller is a tiny computer that receives data through the input components. The data it receives is sent to the CPU through the communication module. This computer then analyzes the data and sends an output or operating instructions to the connected machines. That is the simplest definition you can get about what is a plc controller.

What are the Basics of PLC?

Now that we have answered the question ” what is a plc controller,” you might be curious to know the basics of a PLC.

Well, there are many things you should know once you have satisfied your curiosity when it comes to “what a plc controller is.” The first thing you need to know is how a Programmable controller work 

A plc works straightforwardly. The input is communicated through the communication module when signals are from the input or output device. Once this is done, the CPU receives the information. Upon receiving the report, the next thing the CPU does is analyze the data.


what are the basics of plc


This is where the CPU components such as RAM, ROM memory, and other parts come in handy. The input or information is stored, processed, and analyzed. Once this is done, the CPU sends operational instructions through the communication module to the output. Once the production receives the information, the operating instructions from the CPU will be carried out instantly. Pretty simple, right?

That’s just the simplified method of how a plc works. With the way it works, you can even compare the functions to that of a computer. We will be giving you a comparison between plc and computer pretty soon.

What are the Types of PLC?

Now that we have addressed the question ” what is a plc controller?”, The next thing you should know is the types of plc and their application.


what are the types of plc


You should know that there are different types of plc, and all of them can be classified in different ways from size to module system, programming language, and a host of other things. Below are different classifications of the types of PLCs you should know.

Classification Based on Size

Regarding PLC types based on size, you should know that there are three major types. Below is brief information on the three types based on size. 


plc classification based on size

Small PLC

As the name implies, a small plc is the type of plc you will find in small machines such as motors and many more. This plc is small and perfect for small appliances as they do not perform many functions.

This plc type still has enough memory to store the ladder storage program and much more. They also have communication ports which you might find interesting. The modules in the small plc can be used independently.

Medium-sized PLC

This type of plc performs a lot more functions than the small-sized plc. When it comes to medium-sized plc, you can use them to perform functions or tasks such as controlling several cars and assembly lines. 

Due to its size, it has a much larger memory size than the small-sized plc. Besides the memory size, it has a higher processing speed, expanded communication ports, and many other things. In simplified terms, it beats small plc in everything except portability.

A perfect example of this medium-sized plc is the digital to analogue converters you see around you. Communication ports that communicate with field actuators are also another example.

Large sized PLC

With the size of this plc, you can control giant machines such as power turbines, power plants, whole building blocks, and a lot more.

This is because PLCs like these allow for a different programming language to be written on them. It has the most significant size when it comes to memory and functionality. An advanced digital-to-analogue converter is an excellent example of a large-sized plc.

Classification Based on Hardware Setup.

Since we have answered the question ” what is a plc,” and we have discussed the first classification based on size, the next thing you can use to classify your plc is based on the hardware setup. Below are the various types.


classification on plc based on hardware

Compact PLC

This variety of PLCs utilizes a micro-based controller. This controller contributes to the management of automation responsibilities. This PLC can be found in a variety of machines, including automobiles and motors. It has a built-in programming language with a user-friendly instruction set, making it simple for you to operate the programmable logic controller.

Module PLC

When it comes to classification based on the hardware configuration, this is the second type of classification that can be made.

The user is free to program in any language they choose when using a module PLC, in contrast to the compact PLC, which can only be programmed using the system’s native language. Compared to the close PLC, it has much more functionality. In addition, it comes with a more significant number of extensions and ports, enabling it to support a wider variety of user-specific functions.

You should also be aware of another aspect of the module PLC: the modules can be used independently or apart from the components that have been assembled.

Classification Based on the Power Supply

When classifying PLCs, it is essential to be aware of this classification, even though it does not appear very frequently. PLCs that fall into this category can be divided into two distinct groups: those that contain an internal power supply and those that are powered by an external source.


plc classification based on power supply


PLCs that have their internal power supply typically come with their own DC voltage already installed. Typically, this is constructed for the circuit breaker located within the storage area. It is safe to say that the inbuilt power storage plc is far superior to the external power supply plc because you will get to save space that would be used for conversion if you used external storage. This is one of the main benefits of the inbuilt power storage plc.

What are the Applications of PLC?

Now that you have gotten all the answers you need when it comes to what a plc controller is, you might be curious to know about the application of this mini-computer and where it can be used. Below is a list of the application where you can use this computer.

Industrial Application of PLC

When it comes to industrial applications, there are a lot of places where the plc can be used. You can use the plc to perform various things such as packing and labelling food. Here, the compact PLC is the best type of PLC to use. Apart from packing and labelling food, you can also use it to do a host of other things such as liquid filling systems, transportation systems, paper industries, glass industries, and much more. You can also use oil and gas to monitor the purging procedure when refining the oil.

Power Station Application

Apart from the industry application, you can also apply the plc to the power station or power generating plants. You can deploy them to places such as the innovative grid system, where they can be used to monitor the condition of the power plants. It is also used in power transmission, the automatic line system, and much more.

Is PLCs Safe?

This is one question you might have on your mind regarding PLCs aside from the major question of what is a plc controller. You should know that PLCs are safe and can always use them as long as they are strictly used to perform their essential functions. You do not have anything to worry about when using this mini-computer.


You might wonder about the comparison or difference between a plc and a computer. You should know that there isn’t much difference between the two. It is safe to say that plc is a type of computer as they perform very similar functions, the only difference is one is automated while the other isn’t.