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.