SEE 463 Concept Study Review

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Questions and Answers

Timers inherently have retentive memory

False (B)

What does retentive memory mean?

Retentive memory refers to data that retains its content following a warm restart after a power failure. It is stored in a retentive memory area, which is an area that retains its content after cycling the power when the CPU changes from STOP to RUN. Retentive memory is used to avoid data loss after power failure.

What is a debounce circuit, and why is it needed in PLCs?

By default what is the unit used in timers?

milliseconds (B) Signup and view all the answers

How many Double Integers (DINT) are used in each counter?

3 (D) Signup and view all the answers

Choose the object(s) below that have retentive memory

Counters (C), Latching / Unlatching (A) Signup and view all the answers

When do you use a mix of counters and timers?

when the time needed goes beyond the double integer limit Signup and view all the answers

What is the main difference between a modular PLC and a fixed PLC?

Modular PLCs allow for adding and replacing I/O modules, while fixed PLCs have a set configuration (B) Signup and view all the answers

Which of the following is NOT an advantage of PLCs over relay logic?

Slower response time (C) Signup and view all the answers

Which PLC component is responsible for processing the logic and executing instructions?

CPU (C) Signup and view all the answers

What is the function of an I/O module in a PLC?

To interface with input and output devices (C) Signup and view all the answers

Which programming language uses graphical symbols that resemble electrical relay circuits?

Ladder Logic (C) Signup and view all the answers

What is the difference between sourcing and sinking in DC I/O modules?

Sourcing refers to devices that supply current, while sinking refers to devices that receive current. In PLCs, sourcing inputs provide voltage to the input circuit, while sinking inputs complete the circuit by connecting to ground. Signup and view all the answers

What is a PLC scan cycle, and what are its main steps?

The PLC scan cycle consists of: Self-test: The CPU checks internal components. Input scan: The PLC reads the current state of inputs. Program execution: The PLC processes the logic instructions. Output update: The PLC sets the outputs based on the logic execution. This cycle repeats continuously to ensure real-time operation. Signup and view all the answers

What is the difference between scheduled and unscheduled programs in ControlLogix?

A scheduled program is executed as part of the main PLC task and runs at defined intervals. An unscheduled program exists in memory but does not run unless triggered manually. Signup and view all the answers

Explain what a seal-in circuit (latching circuit) is in PLC logic.

A seal-in (latching) circuit keeps an output energized even after the initiating input is turned off. It typically consists of a relay with a normally open contact that parallels the input switch, keeping the circuit active until another input (like a stop button) deactivates it. Signup and view all the answers

What are the four types of PLC programming languages?

Ladder Logic (LL): Uses graphical symbols similar to relay logic diagrams. Function Block Diagram (FBD): Uses blocks representing functions and connections. Structured Text (ST): High-level text-based language similar to Pascal. Sequential Function Chart (SFC): Graphical programming language for process control. Signup and view all the answers

A relay is a device that allows a low-power signal to control a high-power load.

True (A) Signup and view all the answers

A normally closed (NC) contact allows current to flow when the switch is pressed.

False (B) Signup and view all the answers

A PLC-based system is typically more expensive than a relay-based system in large-scale automation.

False (B) Signup and view all the answers

A PAC (Programmable Automation Controller) is similar to a PLC but has advanced capabilities such as multiple controllers and better communication features.

True (A) Signup and view all the answers

Timers in PLC programming are only used to delay turning on an output

False (B) Signup and view all the answers

What is one key difference between a PC and a PLC?

PLCs are specifically designed for real-time industrial control and automation (B) Signup and view all the answers

Which of the following is NOT a type of input device used in PLCs?

Contactors (C) Signup and view all the answers

Which of the following best describes an electromagnetic relay?

A device that uses a magnetic coil to open or close electrical contacts (B) Signup and view all the answers

Which instruction is used in PLC programming to represent a normally closed (NC) contact?

XIO (Examine If Open) (B) Signup and view all the answers

What is the purpose of the Retentive Timer (RTO) in PLC programming?

It retains its accumulated time even if the power is lost or reset (C) Signup and view all the answers

Explain the difference between a relay and a contactor

A relay is used for switching low-power circuits, while a contactor is designed for high-power applications, such as controlling electric motors. Contactors typically have larger contacts and arc suppression features to handle heavy loads. Signup and view all the answers

What are controller-scoped and program-scoped tags in PLCs?

Controller-scoped tags are global variables accessible across multiple programs in a PLC. Program-scoped tags are local to a specific program and cannot be accessed by others. Signup and view all the answers

What are interlocks in control circuits, and why are they important?

Interlocks prevent unsafe conditions by ensuring that one process must be completed before another can start. For example, in a motor control circuit, an interlock might prevent a motor from running in forward and reverse at the same time. Signup and view all the answers

Explain the difference between ladder logic and structured text in PLC programming.

Ladder Logic (LL): Uses graphical symbols resembling relay circuits, making it intuitive for electrical engineers. Structured Text (ST): A high-level text-based programming language, similar to Pascal, used for complex calculations and logic. Signup and view all the answers

Modular PLCs allow for I/O expansion, whereas fixed PLCs have a set number of I/O points.

True (A) Signup and view all the answers

A selector switch is an output device that controls the flow of power.

False (B) Signup and view all the answers

The scan cycle of a PLC starts with executing the program before reading inputs

False (B) Signup and view all the answers

In ladder logic, multiple outputs should not be programmed in series

True (A) Signup and view all the answers

Uploading a program means transferring it from the computer to the PLC.

False (B) Signup and view all the answers

What are the three main types of tasks in ControlLogix PLCs?

Continuous Task: Runs repeatedly with no defined time interval. Periodic Task: Executes at a defined interval. Event Task: Triggers execution based on an event (e.g., an input change). Signup and view all the answers

How does a Normally Open (NO) contact differ from a Normally Closed (NC) contact?

Normally Open (NO): Does not allow current flow until activated. Normally Closed (NC): Allows current flow until activated, at which point it opens the circuit. Signup and view all the answers

What are the three main types of timers used in PLCs, and how do they function?

On-Delay Timer (TON): Delays turning an output ON after a preset time. Off-Delay Timer (TOF): Delays turning an output OFF after a preset time. Retentive Timer (RTO): Accumulates time and retains the value even if power is lost. Signup and view all the answers

What is a fault routine in PLC programming?

A fault routine is a special program that runs when a fault occurs in the PLC. It helps diagnose issues and can trigger safe shutdown procedures. Signup and view all the answers

Why do PLCs have indicator lights, and what do they typically indicate?

Indicator lights show the operational status of the PLC, such as: Power Light: Indicates if the PLC is powered. Run Light: Shows whether the PLC is actively executing its program. Fault Light: Alerts to errors in the system. I/O Lights: Show the status of inputs and outputs. Signup and view all the answers

What happens if a normally open (NO) push button is pressed in a control circuit?

The circuit closes, allowing current to flow (B) Signup and view all the answers

What does an on-delay timer (TON) do?

Turns the output on after a delay (C) Signup and view all the answers

What is the difference between a source and sink configuration in PLC inputs?

Source Configuration: The input device supplies positive voltage to the PLC. Sink Configuration: The input device provides a ground connection, and the PLC supplies the positive voltage. Signup and view all the answers

Describe the purpose of a safety interlock in an industrial control system

A safety interlock prevents unsafe operations by ensuring that specific conditions must be met before an action can occur. For example, in a motor system, an interlock may prevent the motor from starting if a safety guard is open. Signup and view all the answers

What is the difference between latch (L) and unlatch (U) instructions in PLC programming?

Latch (L) Instruction: Once turned ON, it remains ON until explicitly turned OFF by another command. Unlatch (U) Instruction: Turns OFF a latched output. Signup and view all the answers

How does an analog input differ from a digital input in a PLC?

Analog Input: Reads variable values (e.g., temperature, pressure) within a range. Digital Input: Reads binary states (ON/OFF, HIGH/LOW) Signup and view all the answers

A relay with a normally closed (NC) contact remains open until energized

False (B) Signup and view all the answers

A Programmable Logic Controller (PLC) executes ladder logic programs sequentially, from left to right and top to bottom

True (A) Signup and view all the answers

Solenoid valves are commonly controlled by PLC output modules.

True (A) Signup and view all the answers

A memory bit in a PLC program can only store numerical values.

False (B) Signup and view all the answers

The function of a counter (CTU or CTD) in a PLC is to track the number of times an event occurs.

True (A) Signup and view all the answers

Why do industrial control systems often use redundant PLCs?

Redundant PLCs increase reliability by providing a backup system in case the primary PLC fails. Signup and view all the answers

What is scan time in a PLC, and why is it important?

Scan time is the time a PLC takes to complete one cycle of reading inputs, executing the program, and updating outputs. Lower scan times improve response speed. Signup and view all the answers

Name two advantages of using PLCs over traditional relay-based control systems.

Flexibility: PLCs can be reprogrammed for different tasks without rewiring. Reliability: PLCs have fewer mechanical components, reducing wear and tear. Signup and view all the answers

Which logic gate outputs HIGH (1) only when both inputs are HIGH (1)?

AND (B) Signup and view all the answers

Which gate produces an output that is the inverse of its input?

NOT (C) Signup and view all the answers

Which logic gate is equivalent to an OR gate followed by a NOT gate?

NOR (B) Signup and view all the answers

Which of the following is an advantage of using XOR gates in digital circuits?

They are useful for parity checking and addition (B) Signup and view all the answers

What is the main difference between a contactor and a relay?

Contactors are designed for high-power applications (e.g., motors). Relays are for low-power control circuits. Signup and view all the answers

What happens when a relay coil is energized?

The contacts switch positions (NO to closed, NC to open). Signup and view all the answers

A relay can act as a switch to control a higher voltage circuit using a lower voltage signal.

True (A) Signup and view all the answers

What is the main function of a Main Routine in a PLC program?

It serves as the primary execution routine, calling other subroutines as needed. Signup and view all the answers

What are three common types of routines in PLC programming?

Main Routine: The primary logic execution. Subroutine: A reusable function called from the main routine. Fault Routine: Handles errors and exceptions. Signup and view all the answers

What is the difference between a scheduled and unscheduled program in a PLC?

Scheduled programs execute periodically based on a defined cycle. Unscheduled programs execute only when triggered by an event or interrupt. Signup and view all the answers

Which of the following best describes an unscheduled routine?

Executes only when a specific condition is met (B) Signup and view all the answers

A fault routine in a PLC is only executed when the system encounters a serious hardware failure.

True (A) Signup and view all the answers

Which of the following is NOT a factor when selecting a PLC?

The color of the PLC housing (C) Signup and view all the answers

Why is scan time important when selecting a PLC?

Scan time determines the speed of execution and impacts real-time control applications. Signup and view all the answers

Which factor is most important when selecting a controller for an industrial automation system?

I/O expandability and communication capabilities (C) Signup and view all the answers

What are some key factors to consider when choosing a PLC for a manufacturing system?

Number of I/O Memory capacity Scan time Communication interfaces Operating environment Signup and view all the answers

A PLC with faster scan times is always the best choice for any application.

False (B) Signup and view all the answers

Which of the following can cause issues in large logic networks?

All of the above (D) Signup and view all the answers

Why should complex logic be broken into smaller subroutines in PLC programming?

To improve readability, reduce scan time, and simplify troubleshooting. Signup and view all the answers

What is the main purpose of an event-driven program in a PLC?

It executes only when specific conditions are met. Signup and view all the answers

Flashcards

Retentive Memory

Data that retains content after a warm restart after power failure. Stored in a retentive memory area that retains content after cycling power.

Retentive Memory and Counters

Counters retain their values after a power loss, unlike regular timers.