 Hello everyone. Let us today learn PLC programming method ladder diagrams. So at the end of this session you will be able to examine the basic symbols which are used in ladder diagram for the programming of PLCs. Also you can write small ladder run diagrams for basic logic functions. So before going to learn ladder diagrams let us go through the programming techniques which are used for PLC programming. So first one is the ladder diagrams in which writing a program is just equivalent to drawing a switching circuit. Second method is the sequential functional block diagram method in which graphical blocks are used to write programs. Third one is the instruction list in which series of instructions are used to write programs which is very similar to writing a similar language programs in microventilers. Then sequential function charts. So in this a pictorial representation of systems operations are described by number of specific separate sequentially connected states or steps which are normally represented by rectangular boxes. So it is like a flow chart. There is one more method structural text. So in this series of statements which are separated by semicolons are used to write programs. So this method is very similar to Pascal programming language. Now let us start first program method ladder diagrams. So let us understand first the structure of ladder diagrams. So in the ladder diagram two vertical lines are there which represents power rails and the circuit which you want to implement is connected as horizontal lines between these two power rails and basically this is known as rungs of the ladder diagram. So here you will see that these two lines represents power rails whereas this switch and this motor together makes a rung. So here if the switch is pressed motor will start. If switch is released motor will stop. So this is a simple single rung represent here. Now let us take one example how an electrical circuit is converted in a ladder diagram. So here one example is shown. Here we have a DC input in series with switch and motor. So when the switch is pressed motor should start and when the switch is released motor should stop. So this is for representation only but this is the simple example to understand a small ladder rung. So here one switch is required. So in a ladder diagram this DC input is represented by these two power rails and this switch and motor are connected in series between these two power rails. So this become one rung. So this is a simple way to convert your electrical circuit into a ladder diagram. Now let us understand this rung in more drills. So here between these two power lines multiple rungs are shown. So power always flows from left to right. Also each rung on the ladder diagram defines one specific operation in the control process. You need to always read a ladder diagram from left to right and from top to bottom. A device can appear in more than one rung of a ladder diagram. So this depends again upon the operation which you are going to perform through PLC. So now this rung must always start with at least one input or inputs and must end with at least one output. So here the input is nothing but the control action such as closing a contact switch or input from sensor etc. Whereas this output is a device connected to the output such as motor actuator or lamps or relays for example. So these are the two basic elements of this ladder diagram. Now let us learn some symbols which are used to draw ladder diagrams. So switches are available in two forms normally open normally closed. So here normally open switch is normally open until some object closes it and it is always shown as open. Whereas normally closed is a switch that is normally closed ok there is always a contact between two terminals. Next the output coil. So this is the symbol for output coil. So if the power flows through it then the coil state is on otherwise it is in the off state. There is one more symbol which is used to represent non-relier logic operations that is functional block and the non-relier logic operations such as timers counters are used in normal PLC programming. So this depends on the application or operation which you want to implement. Now let us develop some ladder run diagrams for simple logic functions. So here let us take this example in which we are going to develop a ladder run diagram in which an output is not energized unless two normally open switches are both closed. So here this is the input supply then switch one switch two and LED is connected in series. So when these two switches are closed then and then only this LED will glow otherwise this LED will always off. So if you observe this is a simple AND operation. So let us develop a ladder run diagram to perform this AND operation. So here basically you require two normally open switches and one output coil. So all these are connected in series. So in a ladder diagram you will see that between these two power rails two switches and this output coil all are connected in series. So whenever the switch one is energized you will observe that there is still no connection between these two power rails. The LED will remain off or this load will remain off. Suppose switch two independently energized. By assuming that switch one is normally open. So if you close switch two still there is no connection between these two power rails. So still in this case also LED will be in off state. But consider this one one more case in which both the switches are operated okay energized. So when these two when these both switches are energized there is a path between these two power rails and this LED will change its state. So this output coil will change its state from off to on. So this is a simple ladder run diagram for AND operation in which two switches are connected in series with the output coil. Let us take some time and develop a ladder run diagram for this example two. So in this example two the condition is output is not energized unless either of two or both normally open switches are closed. So take some time and draw a simple ladder run diagram for this operation. So to solve this you need to understand the statement. So if you read this statement carefully you will observe that this is a simple OR operation. So in OR operation you require two switches as they have provided two switches in the specification. So these two switches are placed in parallel and in series with power supply and in this LED. So this placement of switch one and switch two provides you the OR operation. So if either of these two switch changes from normally open to normally close normally open to close state. So for example if switch one is closed the circuit will complete and the LED will glow or instead of switch one switch two is closed still the circuit will complete and in the same in this case also LED will glow. And obviously when the both the switches are closed still there is a there is a closed path. So in this case also LED will glow. Only the case in which both switches are open LED will be off. So here if you observe you require two normally open switches and one output coil which represents LED and these two switches are connected in parallel. So in the ladder run diagram also these two normally open switches are connected in parallel but in series with this output coil. So if the switch mean if the switch one is energized there is a path the output coil which coil will change its states from off to on. If switch two is energized in this case also the output coil will change from off to on state as there is a path between these two powers. So these are the two simple logical operations and and or which we have implemented using two run diagrams. Or we have drawn two run diagrams for these two logic functions. For further reading you can refer this book. Thank you.