PLC PLC Communication Protocols & Networking 2 — Questions and Answers
Question 1: In Modbus TCP, on which default TCP port does the server listen for client connections?
- 502 (Correct answer)
- 102
- 443
- 44818
Correct answer: 502
Modbus TCP uses TCP port 502 as its well-known default port for client-server communication over Ethernet.
Question 2: Which function code in Modbus is used to read the status of discrete output coils?
- FC01 (Correct answer)
- FC03
- FC04
- FC05
Correct answer: FC01
Modbus Function Code 01 (FC01) reads the ON/OFF status of discrete output coils from the server (slave) device.
Question 3: What is the primary purpose of a managed industrial Ethernet switch compared to an unmanaged switch in a PLC network?
- A managed switch provides VLAN segmentation, QoS prioritization, port mirroring, and network diagnostics for better control and security (Correct answer)
- A managed switch supports higher data transfer speeds than unmanaged switches
- A managed switch eliminates the need for IP addressing on PLC devices
- A managed switch provides power to PLC processors via PoE
Correct 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.
Question 4: PROFINET uses which communication channel for time-critical, deterministic I/O data exchange?
- Real-Time (RT) channel using standard Ethernet frames with a high-priority VLAN tag (Correct answer)
- Standard TCP/IP socket connections
- UDP broadcast packets on port 5353
- RS-485 serial communication layer
Correct 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.
Question 5: In a PLC network, what is the purpose of a DMZ (demilitarized zone) between the control network and the enterprise network?
- To isolate the OT control network from the IT enterprise network while allowing controlled data sharing through the DMZ (Correct answer)
- To provide wireless access points for mobile HMI devices
- To host the PLC programming software for remote engineering access
- To perform protocol conversion between Modbus and EtherNet/IP
Correct 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.
Question 6: Which communication standard defines a hierarchical network model for industrial automation, separating field, cell, and plant levels?
- ISA-95 (Purdue Model) (Correct answer)
- IEEE 802.3
- IEC 62443
- ISO/OSI 7-layer model
Correct 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.
PLC PLC Communication Protocols & Networking 2
In Modbus TCP, on which default TCP port does the server listen for client connections?