There are five basic PLC programming languages PLC uses for system controls. They include; Ladder Diagrams,Functional Block Diagram,Instruction List, Structured Text and Sequential Function Charts.
Ladder Logic Diagram is most utilized and common PLC programming language among most industrial control processes. As the name suggest these are diagrams made of horizontal and vertical lines which are analogous to circuit diagrams. The power lines, or rails are similar to vertical sides of a physical ladder with the horizontal lines running across the rails the rungs of the ladder. The ladder diagrams shows the the process control exercise, for example the switching of a motor in series with its switch in symbols without showing the actual location of the components.
The process of drawing the ladder diagrams follows certain conventions, they include;
1. Vertical lines of the diagram represent the power rails where the circuit components are connected and power flows from the left hand vertical across the rung.
2. Each ladder rung defines one operation in the process control.
3. The ladder diagram is read from left to right , meaning as the top rung reads from left to right then the second rung is read from left to right.
4. Each rung starts with an input or inputs and must end with at least one process outputs. The input is used for a given control action such as switching action of a motor ,while the output is used for the device connected to the PLC output such as the motor.
5. Electrical devices shown must be in their normal operating state. For example a Normally Open switch to be in an open state and Normally Closed to be closed symbolically.
6. Any particular device can appear more than once in one ladder, for example a switching device like a relay can switch more than one device. In this case, it is labelled with the same number or letter.
Functional Block Diagram (FBD), as the term suggest ,this is a PLC programming described in terms of graphical blocks. This graphical language depicts signal and data flows through blocks which are instruction units which when executed can yield one or more output values.
A function block is represented as a rectangular block with inputs entering the left side and output obtained from their right. The function block name is typed in the block ,such as AND, with the name of the function block in the system shown above for example Counter1.
The function block inputs and outputs names are indicated within the block at the appropriate input and output points . Cross diagram connectors indicates graphical lines to simplify the drawing avoiding cluttering lines and shows where an output at one point is used as an input to another.
An Instruction List programming is method that can be considered to be the entering of a ladder program using text is the instruction list An instruction list gives programs as a series of instructions, with each instruction on a new line. Each instruction consists of an operator followed by one or more operands, that is, the subjects of the operator.
Sequential function chart (SFC) are used for a pictorial representation of a system whose operation to shows the sequence of events involved in its operation. In Structured Text Programs any operation programs have first to define the data types required to represent data.
Generally the Ladder Diagrams and Functional Block Diagrams remains most common and popular PLC programming method as they involve symbols rather than written codes. These can easily be understood by any learner beginning to learn about PLC programming or professionals in the industrial controls.