SCADA, PLC & HMI MCQ [Free PDF] – 3 Objective Questions with Answers

An HMI is the operator-facing interface to PLC/automation data. It can display graphical process mimic screens, trends, alarms, animations and controls, and commonly communicates over Ethernet or serial industrial links such as RS-485. Key relation: HMI reads/writes tags exposed by PLC/SCADA systems.

Exam focus: Use HMI for visualization and operator commands, not primary hardwired safety functions. Common trap: Modern HMIs are not text-only displays.

Modbus is a widely used industrial protocol with serial variants such as Modbus RTU and Ethernet variant Modbus TCP. RS-232 is intended for relatively short point-to-point links and is not a several-kilometre physical layer without converters/repeaters. Key relation: RS-485 is preferred for longer multidrop serial links compared with RS-232.

Exam focus: Match protocol and physical layer; they are not the same concept. Common trap: RS-232 distance is limited.

SCADA supervises distributed/industrial processes by acquiring data, presenting graphics, alarming, logging and trending. Fast deterministic local interlocking/control is normally executed in PLCs/RTUs, while SCADA coordinates and supervises at a higher level. Key relation: Typical hierarchy: sensors/actuators → PLC/RTU → SCADA/HMI.

Exam focus: SCADA supervises; PLC performs fast local logic. Common trap: Do not replace PLC safety/interlock logic with a SCADA screen.