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Principles of Operation

A programmable controller, as illustrated in Figure 1-5, consists of two basic sections:

    *  The Input/Output interface system
    *  The Central Processing unit

The central processing unit (CPU) governs all PLC activities. The following three components, shown in Figure 1-6, form the CPU:

•  The Processor
•  The Memory System
•  The System Power Supply

OPERATION:

The operation of a programmable 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.

INPUT / OUTPUT:

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.

POWER SUPPLY:

The system power supply provides all the voltages required for the proper operation of the various central processing unit sections.

PROCESSOR:

The processor sequentially reads the inputs, execute the program in memory, and update the outputs.

MEMORY:

The memory is used to store the user program.

PROGRAMMING DEVICE:

Although not generally considered a part of the controller, the programming device, usually a personal computer or a manufacturer’s mini programmer 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.

How the PLC Operates:

Application Environment

Panel

Inputs and Outputs to be Controlled

Logic

Ladder Program

Scan Cycle

PLC Brief - Presentation: