SACA SCADA Systems — Questions and Answers
Question 1: In a SCADA system, what is the primary role of a Remote Terminal Unit (RTU)?
- To host the central historian database for long-term data storage
- To collect field data from sensors and transmit it to the master station (Correct answer)
- To render the graphical Human-Machine Interface for operators
- To execute the main PLC ladder logic program for process control
Correct answer: To collect field data from sensors and transmit it to the master station
An RTU is a ruggedized field device that interfaces directly with sensors and actuators, collects real-time data (e.g., pressure, temperature, valve position), and transmits it over a communication network to the SCADA master station (MTU) for display and logging.
Question 2: Which communication protocol is most commonly associated with legacy SCADA systems communicating with RTUs over serial links?
- EtherNet/IP
- DNP3 (Distributed Network Protocol 3) (Correct answer)
- OPC-UA
- PROFINET
Correct answer: DNP3 (Distributed Network Protocol 3)
DNP3 was designed specifically for SCADA communication between master stations and RTUs, particularly in electric utilities and water/wastewater sectors. It supports time-stamping, data integrity features, and efficient polling over low-bandwidth serial links — features absent from general IT protocols.
Question 3: What distinguishes a SCADA system from a Distributed Control System (DCS)?
- SCADA systems cannot display real-time data; DCS systems can
- SCADA is designed for geographically dispersed monitoring over wide areas; DCS is optimized for tightly integrated continuous process control at a single facility (Correct answer)
- DCS systems use PLCs while SCADA systems use microcontrollers exclusively
- SCADA provides closed-loop control while DCS only provides open-loop monitoring
Correct answer: SCADA is designed for geographically dispersed monitoring over wide areas; DCS is optimized for tightly integrated continuous process control at a single facility
SCADA excels at supervisory monitoring and control over large geographic areas (pipelines, power grids, water distribution) with relatively slow scan rates. DCS is optimized for tightly coupled continuous process control (refineries, chemical plants) at one facility with fast, deterministic control loops.
Question 4: A SCADA historian serves which primary function in an industrial operation?
- It executes control algorithms when the primary PLC fails over
- It stores time-series process data for trend analysis, reporting, and regulatory compliance (Correct answer)
- It provides the graphical operator interface for real-time monitoring
- It translates between Modbus and EtherNet/IP protocols
Correct answer: It stores time-series process data for trend analysis, reporting, and regulatory compliance
A SCADA historian (e.g., OSIsoft PI, Wonderware Historian) is a specialized time-series database that archives process values at high frequency over long periods. This data supports trend analysis, production reporting, equipment diagnostics, and regulatory recordkeeping.
Question 5: In SCADA terminology, what is an HMI's primary function?
- To perform high-speed PLC scan cycles on the process controller
- To provide operators with a graphical interface for monitoring process status and issuing control commands (Correct answer)
- To route SCADA data packets between remote substations and the control center
- To enforce cybersecurity policies by filtering Modbus TCP traffic
Correct answer: To provide operators with a graphical interface for monitoring process status and issuing control commands
The Human-Machine Interface (HMI) is the graphical front-end of the SCADA system. It displays real-time process mimics, trend charts, and alarms, and allows operators to issue commands (e.g., open a valve, start a pump) to the underlying control hardware.
Question 6: Which IEC standard defines communication requirements for substation automation and is widely used in modern power-grid SCADA systems?
- IEC 61511
- IEC 61850 (Correct answer)
- IEC 62443
- IEC 60870-5
Correct answer: IEC 61850
IEC 61850 is the international standard for communication networks and systems in substations. It defines data models and services (GOOSE, Sampled Values, MMS) that enable interoperability between intelligent electronic devices (IEDs) from different vendors in power-grid SCADA environments.
SACA SCADA Systems
In a SCADA system, what is the primary role of a Remote Terminal Unit (RTU)?