This is helpful for us who just come to finish our formation. A programmable Logic Controller (PLC) is an industrial process control computer which has been "ruggedized" and adapted to control the manufacturing process. One cannot see the individual inputs or outputs being energized as the function block works as a whole and is highlighted as a whole element. Below is a sample: Some training may be needed to learn this. Ladder Logic is used widely in primitive to medium PLCs and it is easy to visualize electrical connections provided. Ladder Logic is very simple to program, even an automation maintenance electrician can program or troubleshoot ladder logic. The first generation PLCs were programmed with a technique that was based on relay logic wiring schematics. Figure 1a shows a situation where an output is not energized unless two, normally open, switches are both closed. Thank you for sharing knowledge. Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! Now let’s talk about the six most used logic functions in PLC ladder programming //. Below is a very simple motor control relay logic and its corresponding ladder logic. Logic is the simplest form of algorithm that, via the states of its inputs can set some outputs. Here’s an overview of the most important blocks in the official FBD description.The most basic functionality of a PLC program is logic. These functions combine binary variables to form statements. Function Block Diagram . how do we NOT an input in programming? This eliminated the need to teach electricians, maintenance technicians and engineers how to program. for example if we have a NO contact and we want to NOT it, shall we define it as NC in programming? With no input to input A, the contacts are closed and so there is an output. The content is copyrighted to EEP and may not be reproduced on other websites. Sequential Function Chart is a graphical representation for depicting sequential behavior of a control system. Only when A and B are both on is there an output. Suppose we follow an OR gate by a NOT gate (Figure 8a). This illustrates a NOT gate in that there is an output when there is no input and no output when there is an input (Figure 5c). When both input A and input B are activated, there is no output. However, here we “wire” blocks together, while LD has more of a relays logic. It represents signals or data flow into the function block and when it is executed in the PLC logic, results in one or more outputs. Switch A and switch B have both to be closed, which thus gives an AND logic situation. There is an output when neither input A or input B is 1. Thank you, Nice collection of knowledge and good for the beginner. In general // On a ladder diagram contacts in a horizontal rung, i.e., contacts in series, represent the logical AND operations. Logically there is no any difference in all PLC programming languages , only Programming method and technique is different. We can think of this as representing a control system with two inputs A and B (Figure 1b). Each function block is already pre-programed to do a certain function, the user has to plug in the inputs and outputs. Disadvantage: More rungs of ladder to maintain. The line then terminates with O to represent the output. We can think of this as representing a control system with two inputs A and B (Figure 1b). When input A and input B are not activated then there is 0 output. The ladder diagram starts with | |, normally open contacts labeled input A, to represent switch A and in parallel with it | |, normally open contacts labeled input B, to represent switch B. If any modifications needed or any duplication or re-use of the same code in a different part of the logic is needed, extensive work is required to analyze and modify the changes. Only when A and B are both on is there an output. It is a low overhead language and it is executed faster as compared to other methods of programming a PLC. https://program-plc.blogspot.com/2010/02/logic-ladder-diagram.html In the standard from IEC a lot of function blocks are described. Good! Each function block has predefined pins for input and output. When input A and input B are both not activated, there is a 1 output. Functional block diagram. Such an operation is said to be controlled by a logic gate and the relationship between the inputs to a logic gate and the outputs is tabulated in a form known as a truth table. Boolean (Discrete), Integers, Floating Point, Strings, and Time. To t… Instruction List is a low level, text based language that uses mnemonic instructions or they resemble assembly language programming. An example of a NOT gate control system is a light that comes on when it becomes dark, i.e., when there is no light input to the light sensor there is an output. Figure 9 shows a ladder diagram of a NOR system. Now, you will see how statements that combine two or more of these binary variables can result in either a TRUE or FALSE condition, represented by 1 and 0, respectively. A combination of Relay Ladder Logic and Function Blocks is the best way to program a PLC as it is easier for both the programmer and electricians and maintenance personnel. These blocks can be strung together to complete an entire program. In this example of a logic gate, input A and input B have two sets of contacts in the circuits, one set being normally open and the other normally closed. As programmable logic controllers became more sophisticated it has also been used in very complex automation systems. at first kindly accept my deep appreciation for your great lessons. Tags can be of different data types. For instance, lines on a ladder logic diagram could include opening or closing a switch, turning a motor on or off, or energizing a coil on a relay. Also it gets very tricky for the maintenance engineers to analyze and maintain equipment if they do not know how to work with SFC. But the physical switches and coils of relay logic are replaced with PLC’s memory location which are represented as Inputs (I) and Outputs (O). We compared “AND” and “OR” logic function blocks with their ladder logic counterpart. An alternative, which gives exactly the same results, is to put a NOT gate on each input and then an AND gate for the resulting inverted inputs (Figure 8b). Basic functions which are required to run the process like latching instructions, flip-flops and interlocks designed in function blocks are complicated to troubleshoot as compared to relay ladder logic. The input A contacts are shown as being normally closed. Thus if we use 1 to indicate an on signal and 0 to represent an off signal, then for there to be a 1 output we must have A and B both 1. Such a situation involves the AND logic function, condition A and condition B having both to be realized for an output to occur.