What does PLC consist of?

A typical PLC can be divided into parts, as illustrated in Figure 8 which are

  • Central Processing Unit (CPU)
  • Input/output (I/O)
  • Power Supply
  • Programming Device
Figure 8 Typical parts of a PLC

Figure 8 Typical parts of a PLC

We shall briefly discuss each part here.
Central Processing Unit (CPU)
The processor (CPU) is the “brain” of the PLC. A typical processor usually consists of a microprocessor for implementing the logic and controlling the communications among the modules. The processor requires memory for storing the results of the logical operations performed by the microprocessor. Memory is also required for the program EPROM or EEPROM plus RAM. The CPU controls all PLC activity and is designed so that the user can enter the desired program in relay ladder logic.
The PLC program is executed as part of a repetitive process referred to as a scan (Figure 9). A typical PLC scan starts with the CPU reading the internal state of the CPU. Then, the CPU reads the reading the status of inputs. Whatever program logic has been stored is executed based on the status of the inputs. Once the program execution is completed, the status of all outputs is updated. This process is repeated continuously as long as the PLC is in the run mode.

Figure 9 PLC scan cycle

Figure 9 PLC scan cycle

Input/Output (I/O)
The I/O system forms the interface by which field devices are connected to the controller. The purpose of this interface is to condition the various signals received from or sent to external field devices. Input devices such as pushbuttons, limit switches, and sensors are hardwired to the input terminals. Output devices such as small motors, motor starters, solenoid valves, and indicator lights are hardwired to the output terminals. To electrically isolate the internal components from the input and output terminals, PLCs commonly employ an optical isolator, which uses light to couple the circuits together.
Power Supply
The power supply supplies DC power to the internal circuitry of CPU and other modules that plug into the rack. For large PLC systems, this power supply does not normally supply power to the field devices. With larger systems, power to field devices is provided by external alternating current (AC) or direct current (DC) supplies.
Programming Device
A programming device is used to enter the desired pro-gram into the memory of the processor. The program can be entered using relay ladder logic, which is one of the most popular programming languages. Instead of words, ladder logic programming language uses graphic symbols that show their intended outcome.
A personal computer (PC) is the most commonly used programming device. Most brands of PLCs have software available so that a PC can be used as the programming device. This software allows users to create, edit, document, store, and troubleshoot ladder logic programs. The PC communicates with the PLC processor via a serial or parallel data communications link.

 

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