CAS CAS Automation Specialist Network Communications & Protocols 1 — Questions and Answers
Question 1: Which industrial protocol uses a master-slave architecture over a two-wire RS-485 physical layer and is widely used for connecting PLCs to field devices?
- Modbus RTU (Correct answer)
- EtherNet/IP
- PROFINET
- DeviceNet
Correct answer: Modbus RTU
Modbus RTU operates over RS-485 using a master-slave architecture and is one of the most prevalent serial protocols for PLC-to-field-device communication.
Question 2: EtherNet/IP uses which underlying protocol suite to transport CIP (Common Industrial Protocol) messages?
- TCP/IP and UDP/IP (Correct answer)
- RS-485 and RS-232
- PROFIBUS DP
- CAN bus
Correct answer: TCP/IP and UDP/IP
EtherNet/IP encapsulates CIP messages over standard TCP/IP (for explicit messaging) and UDP/IP (for implicit I/O messaging).
Question 3: What is the maximum cable length for a PROFIBUS DP network segment operating at 1.5 Mbps?
- 200 m (Correct answer)
- 400 m
- 1000 m
- 100 m
Correct answer: 200 m
At 1.5 Mbps, PROFIBUS DP specifies a maximum segment length of 200 m; longer runs require repeaters or a lower baud rate.
Question 4: In an industrial Ethernet network, which device operates at Layer 2 of the OSI model to forward frames based on MAC addresses?
- Managed switch (Correct answer)
- Router
- Repeater
- Gateway
Correct answer: Managed switch
A managed switch operates at OSI Layer 2 (Data Link) and forwards Ethernet frames based on MAC address tables.
Question 5: HART (Highway Addressable Remote Transducer) protocol superimposes a digital signal on top of which analog signal range?
- 4–20 mA (Correct answer)
- 0–10 V
- 0–5 V
- 1–5 V
Correct answer: 4–20 mA
HART uses FSK (Frequency Shift Keying) to overlay digital communication on the standard 4–20 mA analog current loop without disrupting the analog signal.
Question 6: Which network topology is most commonly used in PROFINET IO for its simplicity and ease of troubleshooting?
- Line (daisy-chain)
- Ring
- Star (Correct answer)
- Mesh
Correct answer: Star
Star topology is most common in PROFINET IO, with all devices connected to a central managed switch, making fault isolation straightforward.
Question 7: What is the primary function of the OPC UA (Unified Architecture) standard in industrial automation networks?
- Platform-independent, secure data exchange between automation systems (Correct answer)
- Physical layer specification for fieldbus wiring
- Wireless sensor communication protocol
- Real-time motion control over Ethernet
Correct answer: Platform-independent, secure data exchange between automation systems
OPC UA provides a vendor-neutral, platform-independent framework for secure and reliable data exchange between industrial devices and enterprise systems.
CAS CAS Automation Specialist Network Communications & Protocols 1
Which industrial protocol uses a master-slave architecture over a two-wire RS-485 physical layer and is widely used for connecting PLCs to field devices?