A little about PLC’s

It seems to me that most information available on this topic assumes a moderate knowledge level before beginning their discussion, so I intend to break this down a little to accommodate those of you who may not have even heard of PLC’s before today.  Along the way, I will share my perspective as someone who has been working “hands-on” since the late eighties.
The most basic form of PLC’s are hardened, industrialized PC’s which provide inputs and outputs.

The Inputs

Are information (coming in for use by the processor)in the form of voltages (on or off) which comes from various types of sensors, Such as level indicators for liquid vessels (full or not full) , door switches (closed or not closed), position switches for moving parts (in position X or not), and presence sensors( is a part at position X or not).Also from User Control Panels in the form of selector switches, push buttons, etc.  such as mode,( on-off), speed (fast-slow), desired position (left-right), (up-down), stop, etc. Each device is wired to its own unique input.

The Outputs

Serve a different function, They are controlled by the processor and are meant to manipulate various equipment outside the PLC. Like the inputs, they are either switched on or off depending on what the processor would like to see happen. Each output is wired to its own unique device, and since the processor knows exactly what device is wired to each I/O, The user may manipulate things the way they wish within the user program.


And now we arrive at the wonderful world of programming these processors. Bear in mind we are discussing the most basic scenario for the purpose of gaining a rudimentary understanding of what is going on here. There are variations we have not discussed which are beyond the scope of this article.
These devices do not run the same type of operating system that a PC runs, therefore, the method of programming their processors is different too. Each PLC manufacturer provides a software programming package to be used on a PC that is specific to their own PLC brand.
Typically the programming cables that connect the programming PC to the PLC are also proprietary insofar as pin assignments and connector types are concerned.
The most common type of programming language is called Ladder Logic. It was developed early on to help Industrial Electricians make the transition. Graphically it is similar to a wiring diagram and is really very easy to interpret, So It remains a favorite to this day.

There are usually other languages available within the programming software package that have their own advantages and disadvantages depending on exactly what the programmer is trying to accomplish. The following is a list of typical selections:
Ladder diagram (LD)
Sequential Function Charts (SFC)
Function Block Diagram (FBD)
Structured Text (ST)
Instruction List (IL).

It might be worth mentioning that Europeans seem to prefer structured text whereas in the US we seem to prefer ladder logic. I could never understand that since a picture is worth a thousand words. But to each his own.

Analog Option

Since the intent here is to leave the reader with a basic understanding of the principles, I will mention one other thing the reader probably needs to be aware of. That would be the fact that there are Analog inputs and outputs available whereas up to now we have discussed only digital I/O. (on or off only).
Analog I/O is needed for numerical values that are somewhere between on and off insofar as voltage levels are concerned.
Things like exact temperature, level, height, and distance might be good examples for analog input. Exact valve positioning for gas, air, hydraulic pressure are good applications for analog outputs. Typically you might see these things in control loops where the PLC is required to maintain some value of position, pressure or temperature while some input conditions may vary.
Each analog I/O requires only one I/O which can vary in voltage or current draw. In the case of inputs, the processor interprets the input voltage level as a binary word representing some value for use by the user program. In the case of outputs, the processor puts out a specific voltage on the specified output line that determines the exact position of a physical object.

Some Random Information

The number of ALL I/O available to the processor is totally dependant on what the buyer bought, sometimes there are only a few and other times there are hundreds or even thousands. Just FYI they can also be networked together through industrial LAN networks(and other propriety networks) for the sake of allowing machines on the same production line to work as a team. (and you thought they weren’t sociable) LOL. Just kidding.  An individual PLC may control one machine or it may control an entire group of machines, also it could be the master to a group of slave PLC’s.

I hope you found this interesting.

if you have any questions leave a comment and I will answer as best I can.

Here’s another article you may be interested in:


Some PLC manufacturers:





Wayne Dover,


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