Jawaharlal Nehru Technological University Hyderabad B.Tech IV INDUSTRIAL AUTOMATION Previous Question Paper (October/November - 2020 - R16)

A Programmable Logic Controller (PLC) is a digital computer used for the automation of industrial electromechanical processes. Its functional diagram consists of five primary components: the Central Processing Unit (CPU), Memory, Input/Output (I/O) modules, Power Supply, and Programming Device.

1. CPU: The brain of the PLC, which executes the control program stored in memory, performs logic operations, and manages communication between modules.
2. Memory: Stores the operating system, the user-written control program, and data (status of inputs/outputs).
3. I/O Modules: Provide the physical interface between the CPU and the field devices (sensors and actuators).
4. Power Supply: Converts AC line voltage into the low-voltage DC power required by the CPU and I/O modules.
5. Programming Device: A PC or handheld unit used to write, edit, and upload the ladder logic or other control programs into the PLC memory.

The operation follows a continuous cycle known as the scan cycle: Input Scan, Program Execution, and Output Scan.

I/O modules serve as the bridge between the high-voltage/current field environment and the low-voltage logic circuitry of the PLC processor.

Input Modules: These modules receive signals from field devices such as pushbuttons, limit switches, and sensors. The input module performs three critical functions: isolation, signal conversion, and status indication. It isolates the PLC from electrical noise and voltage spikes, converts high-level AC or DC field signals into logic-level signals (e.g., 5V DC), and typically features an LED indicator to show the status of the input device.

Output Modules: These modules receive control commands from the CPU and drive field devices such as motor starters, solenoid valves, and indicator lamps. They also provide electrical isolation to protect the CPU from field-side faults. Output modules are categorized based on their switching element: relay outputs (suitable for both AC and DC, but slower), transistor outputs (fast switching, DC only), and triac outputs (AC only, fast switching). Like input modules, they include status indicators to verify that the output command has been sent to the field device.