PLC Communication Protocols & Networking Flashcards

In Modbus TCP, on which default TCP port does the server listen for client connections?

Answer: 502

Modbus TCP uses TCP port 502 as its well-known default port for client-server communication over Ethernet.

Which function code in Modbus is used to read the status of discrete output coils?

Answer: FC01

Modbus Function Code 01 (FC01) reads the ON/OFF status of discrete output coils from the server (slave) device.

What is the primary purpose of a managed industrial Ethernet switch compared to an unmanaged switch in a PLC network?

Answer: A managed switch provides VLAN segmentation, QoS prioritization, port mirroring, and network diagnostics for better control and security

Managed switches offer features like VLANs, QoS, SNMP monitoring, and port security that are critical for reliable and secure PLC network design.

PROFINET uses which communication channel for time-critical, deterministic I/O data exchange?

Answer: Real-Time (RT) channel using standard Ethernet frames with a high-priority VLAN tag

PROFINET's Real-Time (RT) channel bypasses TCP/IP and sends Ethernet frames directly with priority tagging to achieve deterministic cycle times for I/O data.

In a PLC network, what is the purpose of a DMZ (demilitarized zone) between the control network and the enterprise network?

Answer: To isolate the OT control network from the IT enterprise network while allowing controlled data sharing through the DMZ

A DMZ sits between the OT and IT networks with firewalls on both sides, allowing data historians and reporting servers to share data without giving IT direct access to PLCs.

Which communication standard defines a hierarchical network model for industrial automation, separating field, cell, and plant levels?

Answer: ISA-95 (Purdue Model)

ISA-95, based on the Purdue Model, defines the hierarchical levels (Level 0–4) of industrial automation networks from field devices up to enterprise systems.