Lecture 3

Which of the following best describes the primary role of a PLC in industrial automation?

• To execute arithmetic operations and manage office tasks such as word processing

• To store and execute control logic that manages inputs (sensors) and outputs (actuators) of machines and processes

• To provide internet browsing and remote communication for factory workers

• To handle high-resolution image rendering and graphic design applications

One of the weak points of PLCs compared to traditional relay-based control is

• High reprogramming flexibility

• High reliability under industrial conditions

• Ability to integrate sensors and actuators easily

• Higher cost despite similar basic functions

Why is a communication module critical in a PLC system?

• It provides connectivity between the PLC and external networks, HMIs, or SCADA systems

• It stores the program instructions permanently

• It ensures backup power in case of failure

• It directly converts analog signals to digital form

Which of the following best represents a PLC input device?

• Motor contactor controlling a conveyor

• Solenoid valve actuator

• Limit switch monitoring machine position

• Alarm siren

For an application requiring fast and frequent low-power DC switching, which output module is the most suitable?

• Relay output

• Transistor output

• Mechanical contactor

• Triac output

You have two start buttons in parallel and three stop buttons in series. One stop button gets disconnected (open circuit). What happens?

• The machine can still start normally

• The PLC automatically bypasses the faulty stop button

• The machine cannot start, because the series path is broken

• The PLC restarts the system with default logic

The 4–20 mA analog standard is widely used in PLC systems because it

• Allows multiple devices to communicate digitally over a single bus

• Ensures permanent program storage in PLC memory

• Is mainly used for switching AC actuators

• Provides a simple and noise-resistant method of transmitting analog values

Which of the following belongs to the category of industrial fieldbus protocols?

• Profibus

• Bluetooth

•  USB

• Wi-Fi

In ladder logic, a normally open (NO) contact behaves as

• Conducting current in its rest state and opening when actuated

• Non-conducting in its rest state and closing when actuated

• A permanent closed path regardless of input state

•  A device that can only be used in output rungs

Why are stop buttons conventionally wired as normally closed (NC) in ladder circuits?

• To reduce the wiring cost of safety circuits

• To guarantee that loss of control voltage will safely stop the machine

• To allow operators to restart quickly after emergency stops

• To minimize the PLC scan cycle time