The PLC consists of the following major components:
- Power Supply: It powers the components of the PLC with DC voltage. To do this, the power supply converts the user's line AC voltage (e.g., 120 volts in the U.S.) to a lower DC voltage (e.g., 24 volts).
- Processor: A solid-state device that implements industrial control functions in manufacturing environments (e.g., production lines, machine tools, robotic equipment), as well as other process control functions.
- Input/Output (I/O): The PLC contains a variety of input and output modules. Input modules detect the status of input signals from various sources such as push buttons, switches and various sensors. On the other hand, output modules are destined to control devices such as relays and lights.
- Communication Modules and Protocols: The communication modules of a PLC help in transferring digital data between the PLC and other devices in the industrial site. To enable this transfer, the communication module implements one or more protocols, both wired and wireless. a non-exhaustive list of industrial protocols used by PLCs includes EtherNet/IP, Profibus, Modbus, Interbus, ProfiNet, etc. PLC communication takes place through built-in ports on the device, such as USB (Universal Serial Bus), Ethernet, RS-232, RS-485, and RS-422 ports. Using these ports, the PLC communicates with software systems, external devices (e.g., sensors, actuators), and other control systems (e.g., SCADA).
- Redundancy: Important industrial grade PLC installations provide some kind of redundancy. Specifically, they contain a shadow PLC system that is destined to replace the primary system in the event of a failure. In order to detect when the redundant system must be activated, the PLC implements a heartbeat mechanism.
- PLC Program: A key element of a PLC device is its control logic, which is programmed and reflected in languages specific to industrial automation and electrical systems, such as ladder diagrams and function block diagrams.