PLC Programming Languages

Types of PLC Programming Language

• Structured Text (ST) programming: a high-level language similar to Pascal, with statements used to assign values to variables.
• Logic Diagram (LD) programming: a graphical language, widely used, with Boolean mnemonics to represent the process before converting into a logic diagram.
• Functional Block Diagram (FBD) programming: a graphical language used in applications involving signal flow between control blocks.
• Instruction List (IL) programming: a low-level programming language, similar to assembly language programming.
• Sequential Function Chart (SFC) programming: a graphical programming method useful for describing sequential type processes.

Instruction Lists (IL)

• A programming method that can be considered as entering a ladder program using text.
• Programs consist of a series of instructions, each on a new line, with an operator followed by one or more operands.
• Mnemonic codes are used, each corresponding to an operator/ladder element, and differ from manufacturer to manufacturer.

Ladder Programs and Instruction Lists

• Whenever a rung is started, it must use a 'start a rung code' (e.g., LD, A, or L).
• The rung must end with an output or store result code (e.g., OUT, =, or ST).
• Examples of codes used in ladder programs and instruction lists, including those from Mitsubishi and Siemens.

Sequential Function Charts (SFC)

• A pictorial representation of a system's operation, showing the sequence of events involved in its operation.
• SFC charts have elements such as steps, transitions, and actions.
• Part of an SFC and its equivalent ladder program.

Structured Text

• A programming language that strongly resembles Pascal.
• Programs are written as a series of statements separated by semicolons (;).
• Assignment statements are used to indicate how the value of a variable is to be changed.
• Conditional statements (IF...THEN...ELSE) are used to execute selected statements when certain conditions occur.

Electromagnetic Control Relays

• The PLC's original purpose was to replace electromagnetic relays with a solid-state switching system that could be programmed.
• Electromagnetic relays make logic decisions but are not designed to handle heavy current or high voltage.

Contactors

• A special type of relay designed to handle heavy power loads that are beyond the capability of control relays.
• Unlike relays, contactors are designed to make and break higher-powered circuits without being damaged.

Motor Starters

• A motor starter is designed to provide power to motors.
• Motor starters are made up of a contactor with an overload relay attached physically and electrically to it.
• Overload relays are designed to meet the special protective needs of motor control circuits.

Manually Operated Switches

• Manually operated switches are controlled by hand, including toggle switches, pushbutton switches, knife switches, and selector switches.
• Pushbutton switches are the most common form of manual control.

Mechanically Operated Switches

• Mechanically operated switches are controlled automatically by factors such as pressure, position, or temperature.
• Examples include mechanically operated limit switches, temperature switches, and pressure switches.

Sensors

• Sensors are used for detecting and often measuring the magnitude of something, converting mechanical, magnetic, thermal, optical, and chemical variations into electric voltages and currents.
• Examples of sensors include proximity sensors, capacitive proximity sensors, light sensors, and ultrasonic sensors.

Output Device Control

• Solenoid valves are output devices controlled by PLCs.
• Solenoid valves are used to control the flow of fluids, air, or other media.
• Solenoids can be normally open (NO) or normally closed (NC).