Jawaharlal Nehru Technological University Hyderabad B.Tech IV INDUSTRIAL AUTOMATION Previous Question Paper (March - 2021 - R16)

In a PLC, input/output (I/O) modules are addressed to allow the processor to identify the specific physical location of a device. Addressing schemes vary by manufacturer but generally follow a hierarchical structure: Rack number, Slot number, and Terminal number.

The addressing format typically takes the form of a rack/slot/terminal address (e.g., I:012/05). The rack number identifies the chassis, the slot number identifies the specific module position within that chassis, and the terminal number identifies the exact screw terminal on the module where the field device is wired. Modern PLCs often use symbolic addressing, where descriptive tags (e.g., 'Start_Button') are mapped to these physical memory addresses, making the program easier to read and maintain.

Input modules map the status of field devices into an input image table in the PLC memory, while output modules receive data from the output image table to control field devices. The processor scans these tables during the I/O update phase of the scan cycle to ensure synchronization between the program logic and the physical field devices.

Communication in a PLC is critical for integrating individual controllers into a cohesive industrial automation system. It enables data exchange between PLCs, Human-Machine Interfaces (HMIs), Supervisory Control and Data Acquisition (SCADA) systems, and intelligent field devices. This connectivity allows for centralized monitoring, remote diagnostics, real-time data logging, and coordinated control across multiple production lines, which improves overall plant efficiency and reduces downtime.

Two common networking options for PLC communications include:

• EtherNet/IP: Based on the standard Ethernet protocol, this is widely used in industrial environments. It provides high-speed, deterministic data transfer and allows for seamless integration between the factory floor and enterprise-level IT networks. It supports both implicit (real-time I/O) and explicit (configuration/diagnostic) messaging.
• Modbus TCP/IP: A robust, open-standard communication protocol that runs over standard Ethernet hardware. It is highly popular due to its simplicity and platform independence, allowing PLCs from different manufacturers to communicate with each other and with various third-party sensors or actuators in a master-slave configuration.