Basics of PLC (Programmable Logic Controller)

What is Programmable Logic Controller (PLC)

Digital Computer usually used for electromechanical processes (i.e control of assembly lines,process control etc…)

Program used to control machine operation are stored in battery backed up or non-volatile memory.

PLC are designed for -

1.Various analog/digital input and output arrangements. 2. Extended temperature range. 3.Immunity to electrical noise. 4.Resistance to vibration and impact.

Working Principle of PLC (Programmable Logic Controller )

Firstly,the input source convert the real time analog electric signals to suitable digital electric signals which are applied to the PLC through the connector rails.

These input signals are stored in the PLC external image memory in a location known as BITS.The CPU will execute this process.

Then,the program instructions are written onto the programming device through symbols or through mnemonics and stored in the user memory.

The CPU fetches these instructions from the user memory and executes the input signals by manipulating,computing,processing them to control the output.

Next the execution results are then stored in the external image memory which control the output device.

The CPU also keeps a check on the output signal and keep updating the contents of the input image memory according to the changes in the output memory.

In addition CPU also performs internal programming functioning like setting and resetting timer also checking user memory.

How PLC works ?

Advantages of PLC?

1.Robust,Can withstand vibration,temperature changes,humidity and noise. 2.Interface for output and input readily available. 3.Easily programmed.Programming language is easy to understand too. 4. Faster response 5.Less maintenance 6.Easier to troubleshoot. 7.Better reliability 8.Remote control and communication capability.

Disadvantages of PLC

1.Error detection can be problematic and require lot of skill 2.Indefinite hold-up time when problem arise.


Great read and really helpful!

Greetings, I go to school for advanced automation and robotics but, the issue I have is that I honestly don’t understand the why and when of plc programming. The instructions are vague and the whole thing gets a bit distant. If anyone can recommend any tutorials I would greatly appreciate it.

Respectfully Ken Adams

Thank you for sharing. This is so helpful.