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PLC Programming Languages

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The term PLC programming language repeated to the method by which the user communicates information to the PLC.

The standard IEC 61131 was established to standardize the multiple languages associated with PLC programming by defining the following five standard languages:

+ Ladder Diagram (LD): A symbolic depiction of instructions arranged in rungs similar to ladderformatted schematic diagrams.

Ladder diagram language is the most commonly used PLC language and is designed to mimic relay logic. The ladder diagram is popular for those who prefer to define
control actions in terms of relay contacts and coils, and other functions as block instructions.
Note how closely the ladder diagram program resembles the hardwired relay circuit.

+ Function Block Diagram (FBD): A graphical depiction of process flow using simple and complex interconnecting blocks.

Functional block diagram programming uses instructions that are programmed as blocks wired together on screen to accomplish certain functions. Typical types of function blocks include logic, timers, and counters. Functional block diagrams are similar in layout to electrical/electronic block diagrams used to simplify complex systems by showing blocks of functionality. The primary concept behind a functional block diagram is data flow. Function blocks are linked together to complete a circuit that satisfies a control requirement. Data flow on a path from inputs, through function blocks or instructions, and then to outputs.
The use of function blocks for programming of programmable logic controllers (PLCs) is gaining wider acceptance. Rather than the classic contact and coil representation of ladder diagram or relay ladder logic programming, function blocks present a graphical image to the programmer with underlying algorithms already defined.
The programmer simply completes needed information within the block to complete that phase of the program.
Figurebelow shows function block diagram equivalents to ladder logic contacts.

+ Sequential Function Chart (SFC): A graphical depiction of interconnecting steps, actions, and transitions.
Sequential function chart programming language is similar to a flowchart of your process. SFC programming is designed to accommodate the programming of more
advanced processes. This type of program can be split into steps with multiple operations happening in parallel branches. The basic elements of a sequential function chart program are shown in Figure below

+ Instruction List (IL): A low-level, text-based language that uses mnemonic instructions.

+ Structured Text (ST): A high-level, text-based language such as BASIC, C, or PASCAL specifically developed for industrial control applications.
Structured text is a high-level text language primarily used to implement complex procedures that cannot be easily expressed with graphical languages. Structured text uses statements to define what to execute.

For this application, the objective is to energize SOL 1 whenever either one of the two following circuit conditions exists:
+ Sensor 1 and Sensor 2 switches are both closed.
+ Sensor 3 and Sensor 4 switches are both closed and Sensor 5 switch is open.

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