Amatrol’s portable Siemens PLC troubleshooting eBooks start with the basics, such as the components of a PLC. Although the capabilities for expansion and function vary from model to model, most PLCs consist of five basic components: processor module, discrete input/output modules, power supply, I/O rack, and programming device.

• Processor Module – The processor module, or controller, handles the PLC’s data processing. It consists of a central processing unit (CPU) and memory. The control program is stored in memory. The controller receives data input, processes information, and executes instructions.
• Discrete Input/Output Modules – S7-1200 PLCs can be connected to digital input modules, digital output modules, or modules that contain both digital inputs and outputs. A discrete input module provides the wiring terminals and circuitry that enables a PLC to sense the state or condition of a machine or application. A discrete output module provides the wiring terminals and circuitry that enable a PLC to control connected output devices.
• Power Supply – The power supply rectifies and regulates AC line power to 24 VDC power for processor and I/O module operation.
• I/O Rack – The I/O rack is typically a DIN rail, which provides a mechanical mounting surface for the controller’s processor, power supply, and various input, output, and communications modules.
• Programming Device – PLCs are most commonly programmed using a standard PC running the appropriate programming software.