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.


Improving Energy Efficiency in Wastewater Treatment Plants

A wastewater treatment plant separates solids from the liquid and consists of two basic stages: primary treatment and secondary treatment.

In the primary treatment stage, larger solids are removed from wastewater by settling.

Secondary treatment is a biological process for further removal of the remaining suspended and dissolved solids through a biological process of adding microorganisms to the wastewater. This process is accomplished in an aeration basin.

The purpose of this white paper is to contribute to a better understanding of how to improve energy efficiency in wastewater facilities, it is known for a fact that one of the biggest expenses in wastewater treatment operations is the cost of energy to run the blowers and compressors that produce air for the aeration basins. Improvements in energy efficiency allow the same work to be done with less energy.

The figures most often cited are that 40 to 50 per cent of a wastewater plant’s total energy usage can be attributed to the aeration process. By measuring the system’s air flows with an accurate, repeatable Dissolved Oxygen sensor, the aeration process can be better controlled to optimize the process and minimise plant energy costs. Especially with the increasing costs of energy which is a major concern of plant operators. Typically, treatment plants have a high potential for energy savings.

 Dissolved Oxygen Measurement is a means to improve energy efficiency

 The organic wastes entering a wastewater treatment plant are broken-down using biomass. The biomass must be kept alive by maintaining the proper dissolved oxygen concentration throughout the treatment process.

 The ability to maintain the proper concentration of Dissolved Oxygen in an aeration basin is necessary to keep microorganisms alive for a breakdown of the organic waste.

A precondition for effective measures is built around the knowledge about the process characteristics of the individual plant sections.

Sewage that flows into a treatment plant is sent to a Primary clarifier tank to remove the garbage and sludge. Thereafter, it is routed to an Aeration basin where biomass is added and oxygen is blown into the mixture to agitate it. The microorganisms in the biomass thrive on oxygen and cause the organic substances in the sludge to proliferate and the generated solids to be precipitated.

In the absence of enough dissolved oxygen, at least some of the biomass will die, and the organic wastes will not be properly treated. Contrarily, when there is too much-dissolved oxygen, usually from over aerating, the wastewater treatment plant is needlessly wasting costly energy.

Monitoring dissolved oxygen is an essential part of the activated sludge process. It assists in ensuring that there is sufficient dissolved oxygen in the process for the biological activity to take place, and helps to optimize energy usage through controlled air addition.

Maintaining a proper level of dissolved oxygen can only be accomplished by using an online instrument such as the Endress+Hauser On-line Dissolved Oxygen (DO) sensor that will accurately and continuously measure the dissolved oxygen concentration at various stages throughout the treatment process.

Equipping the aeration basin with online DO measurement automates the aeration system to maintain the correct amount of DO. With the presence of the Dissolved Oxygen sensor and the monitoring system in the Aeration tank:

  • Oxygen levels are controlled by comparing the measured oxygen concentration and the oxygen set-point
  • Optimized blower usage for reduced energy consumption which leads to a significant reduction in energy costs for the wastewater treatment plant


 Greenpeg Engineering is one of the leading industrial, distribution and service companies in Nigeria that aims to revolutionize the industry through offerings in unrivalled product bouquet and engineered industrial solutions, applied by seasoned engineers with first-class technical know-how.

Our industrial services cut across industries such as Food & Beverages, Oil & Gas, Primary (mining, metals, cement), Power & Energy, Chemicals, Petrochemicals, Water & Wastewater, Life Sciences, Pulp & Paper and Renewable Energy industries.


13 Things to Consider Before Choosing Your PLC 

The importance of programmable logic controllers (PLC) across the manufacturing industry cannot be overemphasized. If there is one thing you should know: these mini-computers relevance goes beyond the manufacturing industry. They are now used in power, automobiles, and many more industries. Many companies manufacture quality and top-notch PLCs, such as Schneider Electric.

If there is one thing you might be curious about, it is your PLC selection criteria. You might be curious about what you need to consider when picking your own PLC. 

You do not need to worry anymore, and we will discuss the perfect plc selection criteria for you. We will also give you the things you need to consider when selecting your plc.

Things to Consider when Choosing your PLC

Regarding PLC selection criteria, you will have to consider many things. Below is a list of the things you should consider when choosing your PLC.


This is the first thing you need to consider if you are looking for a PLC selection criteria for your business. You should pay a lot of attention to the size of the plc. PLC comes in different sizes, and you should pick the one that is just okay for the purpose you need it for.


consider the plc size


Regarding size as a plc selection criterion, you must ask yourself several questions. ” How big do I want my PLC? What are the functions the size can accommodate? Based on the location of the PLC, do I need a large or a small-sized PLC? All these questions play a vital role in PLC selection criteria.

You need to consider the size, especially if you move the plc from one place to another.

CPU Speed

It would be best if you considered this. It would be best to consider this when considering the plc selection criteria. You need to consider the CPU speed and how fast you want your plc to process information or give outputs.

Consider the PLC Speed

There are different PLCs in the market today, each with a different CPU speed. The CPU speed largely influences the functions of the plc. If you need the plc for a large-scale business such as power transmission, you need a plc with a fast CPU speed.

Memory Capacity

Just as there are different types of plc regarding sizes and CPU speed, you should also know that there are various PLCs regarding memory capacity. Memory capacity is something you should consider necessary when you are looking at a plc selection criteria.

Consider PLC capacity

Does the plc I won’t have enough memory in its RAM and ROM? Do I need to get external storage to the plc? Is there enough space in the plc? It would be best if you asked yourself all these questions regarding the plc selection criteria.


When it comes to PLCs, some exhibit some level of redundancy; there are also some whose processing speed is much faster than the pieces of equipment connected.

PLC Redundancy

You have to consider the redundancy rate regarding the plc selection criteria. What is the redundancy rate? Does the rate have to be reduced or enhanced? It would be best to consider all of these when choosing your plc.

Communication Technique

Another thing you must consider critical when looking at different plc selection criteria. It would be best if you took your time to know the plc communication technique or communication processes. 

How does the input method work? Which device or equipment does it communicate with? What are the communication protocols? All these questions are pretty necessary when it comes to communication protocols as a plc selection criteria.


PLCs come with different I/Os, and you should pay a lot of attention to them when you want to buy one for yourself. Regarding the PLC selection criteria, the I/O is relevant as it affects the mode of your operation.

Consider the input and output of the plc

You need to know how many devices or equipment you plc will or can control. You also need to confirm if any requirement is necessary for it to function with remote I/O interfaces.

Power Method

You should also consider the power method regarding the PLC selection criteria. You need to confirm if the plc has an inbuilt power source or if it has an external power source.

Some people prefer PLCs with an inbuilt power source, and others prefer an external power source when it comes to PLCs. Regardless of the one you like, do well to check the type of power source it uses when it comes to PLC selection criteria.


If you want to get your plc, you need to ascertain the compatibility of the plc. You need to know if the plc will be built from scratch. If it is being made from scratch, will it be uniquely created just for the purpose you want to use it for, or can you use it for other purposes?

Will the plc be able to interface with existing software or hardware? Are they compatible with just any device? All these questions are things you need to ask regarding compatibility as a plc selection criteria.


When it comes to production, the output hardly remains static. This is why you need to consider scalability regarding PLC selection criteria. If the business needs expansion, can the PLC accommodate the expansion?

Can the plc accommodate new devices being added to it? If there is a need to reduce or scale down, can the plc be reprogrammed to do such? Scalability is essential when it comes to plc selection criteria. 

This is because you can determine the amount of output you need for the production process as long as the plc is scalable.

External Environment

The external environment where the PLC will be placed is one thing that plays a vital role in plc selection criteria. Some settings are relatively harsh, and you would likely need particular casing types if you use PLCs in such an environment.

Consider the PLC environment

It would be best if you asked yourself the environment you will be using the plc. You also need to ask if your plc will need any protective gear or more once you look at the background.

Tasks / Actions

Some advanced PLCs can often perform individual actions such as robotics, safety, etc. If you want your plc to perform these actions, you must consider the tasks it can do when choosing a place selection criteria.

Although, the more advanced the action, the greater the cost. It would be best if you considered the exact purpose or reason for getting the plc.

Task and action the PLC will perform

If you want a simple or basic plc, you will not enjoy things such as motion control and a host of other things. However, If you’re going to enjoy motion control and other things, you must consider it when looking at the plc selection criteria.


Nobody likes to get something that would stop working after a couple of months, and you need to consider the reliability and durability of the Plc the moment you are looking at the selection criteria.

Will plc last for a long time? What is the workload? Is there any guarantee or warranty? What happens if it develops a fault? 

consider the scalability of the plc

All these questions must be asked when looking at the plc selection criteria. The durability of the Plc will go a long way in determining how valuable the plc can be over a long time. 

It doesn’t just determine the usefulness; it also determines how well you get the value of your money.


This is the most important thing to consider when choosing a plc selection criteria. It would be best if you thought about the cost of the plc you would like to get for yourself.

If there is one thing that is obvious about PLCs, it is that they come at various prices. Your budget will decide the plc you can buy regarding PLC selection criteria.

If a brand-new plc is too expensive for you, you can choose a fairly used one. However, they usually do not come with any guarantee. Regarding PLC selection criteria, the plc cost is the first thing or measure that should be on your list.

How Long Does a PLC Last?

PLCs are long-lasting and can easily continue functioning for up to 20 years or even longer, provided they are correctly maintained and occasionally upgraded. Let’s have a look at how.

Make sure that you keep it neat and clean.

The dust produced as a byproduct of industrial processes may contain corrosive or conductive components. To avoid potentially hazardous buildup, dust and particles should be removed regularly using bottled air.

Put the PLC away and inspect the air filters in the ventilation system.

Install the PLC in an enclosure that has a low level of dust. You can prevent it from overheating by often replacing the ventilation filters and ensuring that air can flow freely throughout the enclosure.

Verify your various power sources.

Be sure that the power source you are using does not result in any short circuits or surges of energy and that there is a steady flow of power. In places with significant vibration levels, it is important to check that all sockets, plugs, and terminal strips are connected safely and securely.

Put an end to the shaking.

Verify that the screws and bolts that are used to secure the I/O modules, as well as the PLC system itself, are in place. Check that they are in good shape and fastened securely enough to avoid rattling.

Protect against EMI and RFI interference.

Both electromagnetic interference (EMI) and radio frequency interference (RFI) have the potential to disrupt the operation of a programmable logic controller (PLC). Shield your PLC or position it, so it is not near any sources of interference.

EMI and RFi interface of the PLC


When it comes to plc selection criteria, there are a lot of things you should consider. These things ranging from cost to size and a lot more will help you determine the type of plc you should get for yourself.

All the selection criteria you should consider have been discussed in this article. With a simple read-through, you will get all the necessary information.