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PLC Communication Protocols & Networking Flashcards

6 cards from real PLC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 6 PLC Communication Protocols & Networking flashcards as text

  1. What does the acronym SCADA stand for in industrial control systems?

    Answer: Supervisory Control and Data Acquisition

    SCADA stands for Supervisory Control and Data Acquisition, a system that collects real-time data from field PLCs and RTUs for centralized monitoring and control.

  2. In EtherNet/IP, what is the difference between Class 1 (I/O) messaging and Class 3 (explicit) messaging?

    Answer: Class 1 is cyclic/implicit real-time I/O data exchange; Class 3 is on-demand explicit messaging for configuration and diagnostics

    Class 1 (Implicit/I/O) messaging sends cyclic real-time data via UDP for control; Class 3 (Explicit) messaging uses TCP for on-demand configuration, parameter reads, and diagnostics.

  3. What is the function of a PLC's built-in DHCP server capability on an EtherNet/IP network?

    Answer: To automatically assign IP addresses to connected I/O adapter modules that are configured for DHCP addressing

    Some PLCs can act as DHCP servers to automatically assign IP addresses to remote I/O adapters and other devices configured to receive addresses dynamically.

  4. Which industrial protocol is most commonly used for communication between PLCs and SCADA systems over wide-area networks, particularly in utilities?

    Answer: DNP3 (Distributed Network Protocol 3)

    DNP3 is widely used in electric utility and water/wastewater SCADA systems for reliable, bandwidth-efficient communication between master stations and remote PLCs/RTUs.

  5. When configuring a PLC for Modbus TCP communication, what is the significance of the 'Unit Identifier' (slave ID) byte in the Modbus TCP frame?

    Answer: It identifies the specific Modbus slave device when the TCP connection passes through a gateway that bridges to a serial Modbus RTU network

    The Unit Identifier allows a Modbus TCP gateway to forward the request to the correct serial Modbus RTU slave device on a downstream RS-485 network.

  6. What is the purpose of network redundancy protocols like RSTP (Rapid Spanning Tree Protocol) or DLR (Device Level Ring) in industrial PLC Ethernet networks?

    Answer: To automatically restore network communication within milliseconds when a cable or switch fails, preventing loss of PLC control

    RSTP and DLR detect network failures and reroute traffic through redundant paths within milliseconds, maintaining continuous PLC I/O communication during a single fault.