PLC PLC Communication Protocols & Networking 1 — Questions and Answers
Question 1: Which industrial protocol uses a master/slave architecture over RS-485 serial communication and is widely supported by PLCs for connecting field devices?
- Modbus RTU (Correct answer)
- EtherNet/IP
- PROFINET
- DeviceNet
Correct answer: Modbus RTU
Modbus RTU operates over RS-485 using a master/slave model and is one of the most widely supported serial protocols for PLC-to-field device communication.
Question 2: EtherNet/IP is based on which application-layer protocol that maps industrial objects to standard Ethernet/TCP-IP?
- Common Industrial Protocol (CIP) (Correct answer)
- Modbus TCP
- PROFINET IO
- OPC-UA
Correct answer: Common Industrial Protocol (CIP)
EtherNet/IP uses the Common Industrial Protocol (CIP) as its application layer, running CIP messages over standard TCP/IP and UDP/IP Ethernet.
Question 3: In PROFIBUS DP, what is the role of the 'master' device?
- To initiate all data exchanges and poll slave devices for their input/output data cyclically (Correct answer)
- To provide power to all slave devices on the bus
- To store the program logic for all connected PLCs
- To perform address assignment for new slave devices automatically
Correct answer: To initiate all data exchanges and poll slave devices for their input/output data cyclically
A PROFIBUS DP master controls bus access, cyclically requests I/O data from slaves, and sends output data to them.
Question 4: What is the maximum number of nodes (devices) supported on a single PROFIBUS DP segment?
- 126 (Correct answer)
- 64
- 256
- 32
Correct answer: 126
A single PROFIBUS DP segment supports up to 126 nodes (addresses 0–125), with repeaters required to extend beyond one segment.
Question 5: Which PLC networking protocol is specifically designed for real-time deterministic communication in motion control applications and uses a dedicated ring topology?
- EtherCAT (Correct answer)
- Modbus TCP
- OPC-UA
- DNP3
Correct answer: EtherCAT
EtherCAT (Ethernet for Control Automation Technology) achieves deterministic real-time performance using a ring topology where frames are processed on the fly by each node.
Question 6: What does OPC-UA provide that traditional OPC (OPC Classic) did not?
- Platform-independent, service-oriented communication with built-in security and no dependency on COM/DCOM (Correct answer)
- Faster polling rates for real-time I/O data
- Direct programming access to PLC ladder logic
- Physical layer specification for serial cabling
Correct answer: Platform-independent, service-oriented communication with built-in security and no dependency on COM/DCOM
OPC-UA is platform-independent (not Windows COM/DCOM dependent) and includes security (encryption, authentication) and a flexible information model natively.
PLC PLC Communication Protocols & Networking 1
Which industrial protocol uses a master/slave architecture over RS-485 serial communication and is widely supported by PLCs for connecting field devices?