Programmic Logic Controller

Programmic Logic Controller

Programmable Logic Controller is an industrial-grade digital computer designed to perform control functions—especially for industrial applications.

A PLC is an industrial computer which has been adapted for the control of manufacturing processes such as assembly lines, production lines, robotic devices or any activity that requires high reliability control and ease of programming and process fault diagnosis.

Programmable Logic Controller
The majority of PLCs today are modular, allowing the user to add an assortment of functionality including discrete control, analog control, PID control, position control, motor control, serial communication, and high-speed networking. Compared to older technologies, the PLC is easier to troubleshoot, more reliable, more cost-effective, and far more versatile.

Basic Components

A PLC is composed of a few basic parts. These include a power supply; a central processing unit, or CPU; input/output cards; and a backplane, carrier, or rack that these parts are placed into. The backplane, as shown in Figure 2, creates an electrical connection between all of the separate components, giving the PLC its modular design. This electrical connection includes both power and communication signals. Many PLC manufacturers use proprietary communication protocols on the backplane so that I/O can securely talk to the CPU.

The Power Supply

The power supply provides either 125VAC or 24VDC depending on the application and the circumstances of the installation. As mentioned above, this voltage is bussed down the backplane providing power for the CPU and I/O modules, which come in the form of “cards”. These cards can quickly be added or removed from their slot in the carrier.

The Programming Device and Human-Machine Interface

Outside of the PLC itself are two very important components: the programming device and the human-machine interface (HMI). The programming device can be a desktop computer, laptop, or hand-held instrument from the same manufacturer. There are also fixed I/O PLCs with built-in displays and buttons that allow programs to be written directly on the PLC.

PLCs were first developed in the automobile manufacturing industry to provide flexible, rugged and easily programmable controllers to replace hard-wired relay logic systems. Since then, they have been widely adopted as high-reliability automation controllers suitable for harsh environments.