PLC - Principles of operation

 

A programmable logic controller, as illustrated beow, consists of two basic sections:

• the central processing unit
• the input/output interface system

 

Programmable controller block diagram

The central processing unit (CPU) governs all PLC activities. The following three components, shown in below, form the CPU:

• the processor
• the memory system
• the system power supply

 

Block diagram of major CPU components

The operation of a programmable logic controller is relatively simple. The input/output (I/O) system is physically connected to the field devices that are encountered in the machine or that are used in the control of a process. These field devices may be discrete or analog input/output devices, such as limit switches, pressure transducers, push buttons, motor starters, solenoids, etc. The I/O interfaces provide the connection between the CPU and the information providers (inputs) and controllable devices (outputs).

During its operation, the CPU completes three processes: (1) it reads, or accepts, the input data from the field devices via the input interfaces, (2) it executes, or performs, the control program stored in the memory system, and (3) it writes, or updates, the output devices via the output interfaces. This process of sequentially reading the inputs, executing the program in memory, and updating the outputs is known as scanning. Figure below illustrates a graphic representation of a scan.

 

Illustration of a scan

The input/output system forms the interface by which field devices are connected to the controller (see Figure below). The main purpose of the interface is to condition the various signals received from or sent to external field devices. Incoming signals from sensors (e.g., push buttons, limit switches, analog sensors, selector switches, and thumbwheel switches) are wired to terminals on the input interfaces. Devices that will be controlled, like motor starters, solenoid valves, pilot lights, and position valves, are connected to the terminals of the output interfaces. The system power supply provides all the voltages required for the proper operation of the various central processing unit sections.

 

Input/output interface

 

Although not generally considered a part of the controller, the programming device, usually a personal computer or a manufacturer’s miniprogrammer unit, is required to enter the control program into memory. The programming device must be connected to the controller when entering or monitoring the control program.

 

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