When using a PLC, it’s important to design and implement concepts depending on your particular use case. To do this we first need to know more about the specifics of PLC programming. A PLC program consists of a set of instructions either in textual or graphical form, which represents the logic that governs the process the PLC is controlling. There are two main classifications of PLC programming languages, which is further divided into many sub-classified types.

1. Textual Language
   • Instruction list
   • Structured text
2. Graphical Form
• Ladder Diagrams (LD) (i.e. Ladder Logic)
• Function Block Diagram (FBD)
• Sequential Function Chart (SFC)

Due to the simple and convenient features, graphical representation are much preferred to textual languages.

Ladder Logic

Ladder logic is the simplest form of PLC programming. It is also known as “relay logic”. The relay contacts used in relay controlled systems are represented using ladder logic.
The below figure shows the simple example of a ladder diagram.

In the above-mentioned example, two push buttons are used to control the same lamp load. When any one of the switches is closed, the lamp will glow. The two horizontal lines are called rungs and two vertical lines are called rails. Every rung forms the electrical connectivity between Positive rail (P) and Negative rail (N). This allows the current to flow between input and output devices.

Functional Block Diagrams

Functional Block Diagram (FBD) is a simple and graphical method to program multiple functions in PLC. PLCOpen has described using FBD in the standard IEC 61131-3. A function block is a program instruction unit which, when executed, yields one or more output values. It is represented by a block as shown below. It is represented as a rectangular block with inputs entering on left and output lines leaving at the right. It gives a relation between the state of input and output

The advantage of using FBD is that any number of inputs and outputs can be used on the functional block. When using multiple input and output, you can connect the output of one function block to the input of another. Whereby building a Function Block Diagram.

The figure below shows various function blocks used in FBD programming.

The figure below shows a ladder diagram and its function block equivalent in Siemens notation.

The figure below shows a ladder diagram and its function block equivalent in Siemens notation.

Structured Text Programming

Structured text is a textual programming language that utilizes statements to determine what to execute. It follows more conventional programming protocols but it is not case sensitive. A series of statements (logic) is constituted of expressing assignments and relationships using several operators. The structures text operators are listed below in the image.