IEC 61131-3:2013 - PLC Programming Professional Free Practice Test — 29 Questions

Question 1

Consider a Programmable Logic Controller (PLC) executing a program that utilizes the `TON` function block. The `IN` input of the `TON` is pulsed TRUE for 300 milliseconds and then immediately set to FALSE. The `PT` parameter of the `TON` is configured to 500 milliseconds. Following this sequence, the `IN` input is then set to TRUE again after a brief pause. What will be the state of the `ET` (Elapsed Time) and `Q` (Output) of the `TON` function block immediately after the initial 300-millisecond pulse, and what will be the state of `ET` and `Q` when the `IN` input is subsequently set to TRUE again after the pause?

Accepted Answer

Immediately after the initial pulse: ET = 0 ms, Q = FALSE. When IN is set to TRUE again: ET starts counting from 0 ms, Q remains FALSE until ET reaches 500 ms.

Question 2

A process control system utilizes a `TON` function block to manage a critical heating element. The preset time (`PT`) for this timer is configured to 500 milliseconds. The input signal (`IN`) to the `TON` block is controlled by a sequence of events: it is activated (TRUE) for 300 milliseconds, then immediately deactivated (FALSE) for 100 milliseconds, and then reactivated (TRUE) for a continuous duration of 700 milliseconds. What will be the state of the `Q` output of the `TON` function block at the conclusion of the second activation period of the `IN` signal?

Accepted Answer

TRUE

Question 3

Consider a scenario where a PLC programmer is tasked with manipulating a status word represented by the WORD data type. The initial value of this status word is `16#A5B3`. The programmer needs to implement a function that rotates the bits of this status word five positions to the left. What will be the resulting hexadecimal value of the status word after this operation?

Accepted Answer

16#B66A

Question 4

Consider a Programmable Logic Controller (PLC) executing a program that utilizes the `TON` (Timer On Delay) function block as defined by IEC 61131-3:2013. The function block is configured with a preset time (`PT`) of $1000$ milliseconds. The input signal (`IN`) to this `TON` block is asserted (TRUE) for $500$ milliseconds and then immediately de-asserted (FALSE). What will be the state of the timer\'s elapsed time (`ET`) and its output (`Q`) immediately after the `IN` signal transitions from TRUE to FALSE?

Accepted Answer

`ET` will be $0$ milliseconds and `Q` will be FALSE.

Question 5

Consider a scenario involving a conveyor belt system controlled by a Programmable Logic Controller (PLC) programmed according to IEC 61131-3:2013. A `TON` function block, named `ConveyorRunTimer`, is used to ensure the conveyor runs for a minimum duration before a subsequent process step is enabled. The `PT` (Preset Time) of `ConveyorRunTimer` is set to 500 milliseconds. The `IN` input of `ConveyorRunTimer` is connected to a proximity sensor detecting the presence of a product on the conveyor. If the product is detected for exactly 200 milliseconds before the sensor loses detection (i.e., `IN` transitions from TRUE to FALSE), what will be the state of the `Q` output and the `ET` output of the `ConveyorRunTimer` function block immediately after the `IN` signal becomes FALSE?

Accepted Answer

Q is FALSE, ET is 0

Question 6

Consider a scenario in a manufacturing process where a conveyor belt is intended to run for exactly 5 seconds to transport a component. A `TON` function block is programmed with its `PT` set to 5000 milliseconds. The input signal `IN` to this `TON` block is controlled by a momentary push button that is pressed for only 2 seconds. What will be the state of the `Q` output and the `ET` parameter of the `TON` function block immediately after the push button is released?

Accepted Answer

The `Q` output will be FALSE, and the `ET` parameter will be reset to 0 milliseconds.

Question 7

Consider a PLC program utilizing the `TON` function block. The timer\'s preset time (`PT`) is configured to $500$ milliseconds. The timer\'s input (`IN`) is asserted as TRUE for $300$ milliseconds, after which it immediately transitions to FALSE. What will be the state of the timer\'s elapsed time (`ET`) and its output (`Q`) immediately following the transition of `IN` to FALSE?

Accepted Answer

ET = $0$ ms, Q = FALSE

Question 8

Consider a complex industrial automation system where a single global variable list (GVL) named `SystemParameters` is declared and contains a Boolean variable `EmergencyStopActive`. This variable needs to be monitored and potentially acted upon by multiple independent Program Organization Units (POUs), including several Function Blocks controlling different machine axes and a main Program block responsible for overall system sequencing. What is the fundamental principle of IEC 61131-3:2013 that allows `EmergencyStopActive` to be directly accessed and modified by all these disparate POUs without explicit parameter passing between them?

Accepted Answer

The inherent scope of variables declared within a Global Variable List (GVL) provides universal accessibility across all POUs in the project.

Question 9

Consider a Programmable Logic Controller (PLC) executing a program with a consistent scan cycle time of 20 milliseconds. A `TON` (Timer On Delay) function block is utilized, configured with a Preset Time (`PT`) of 500 milliseconds. If the input (`IN`) to this `TON` function block is set to TRUE at the precise start of a PLC scan cycle, at which point in the PLC\'s execution sequence will the timer\'s output (`Q`) transition to TRUE?

Accepted Answer

At the end of the 25th scan cycle following the activation of the IN input.

Question 10

Consider a scenario in a process control application where a `TON` function block is configured with a preset time (`PT`) of 5 seconds. The `IN` input of this `TON` block is connected to a sensor that is expected to remain active for an extended period. However, during a test, the `IN` signal is activated for only 3 seconds before being de-activated. Simultaneously, a separate `MC` (Master Control) signal, which is not directly wired to the `TON` block\'s enable or reset inputs, is active throughout this period. What will be the state of the `Q` output of the `TON` function block immediately after the `IN` signal is de-activated?

Accepted Answer

FALSE

Question 11

Consider a PLC program structured with a single task executing cyclically. Within this task, a boolean variable `InputSignal` is initialized to TRUE and never subsequently modified. An instance of the `R_TRIG` function block, named `MyTrigger`, is declared and its `CLK` input is continuously assigned the value of `InputSignal`. A separate boolean variable, `OutputState`, is intended to reflect the state of `MyTrigger.Q`. What will be the final value of `OutputState` after the program has executed for several scan cycles?

Accepted Answer

FALSE

Question 12

Consider a scenario involving a conveyor belt system controlled by a PLC. A `TON` function block, named `ConveyorRunTimer`, is configured with a preset time $PT$ of $T\#5s$. The `IN` input of this timer is connected to a proximity sensor that detects the presence of a product on the belt. The `Q` output of the timer is used to activate the conveyor motor. The `IN` signal is TRUE when a product is detected and FALSE otherwise. The sequence of events is as follows: the sensor detects a product for 2 seconds, then loses detection for 3 seconds, and then detects the product again for 5 seconds. At the exact moment the sensor detects the product for the second time, the timer begins its count. What is the state of the `Q` output of the `ConveyorRunTimer` function block at the conclusion of the second detection period?

Accepted Answer

TRUE

Question 13

Consider a function block, `FB_DataHandler`, designed to process and potentially modify a critical sensor reading. This function block utilizes a parameter declared as VAR_IN_OUT to receive and return the processed sensor value. If the calling program segment passes a variable `SensorValue_Raw` to this VAR_IN_OUT parameter, and within the `FB_DataHandler`\'s execution, the parameter is assigned a new value, what is the direct consequence of this assignment on the `SensorValue_Raw` variable in the calling program segment during the same PLC scan cycle?

Accepted Answer

The `SensorValue_Raw` variable in the calling program segment is immediately updated with the new value assigned to the function block's VAR_IN_OUT parameter.

Question 14

Consider a Function Block named `CycleCounterFB` with a single internal variable `Count` of type `INT`, initialized to 0. This `CycleCounterFB` is called once per scan cycle within the PLC\'s main task. If, during the execution of the main task, the `CycleCounterFB` is called and its internal logic increments `Count` by 1, what will be the value of `Count` at the beginning of the *next* scan cycle, assuming no explicit reset mechanism is activated for `CycleCounterFB` and the main task continues to execute normally?

Accepted Answer

The value of `Count` will be 1 greater than its value at the end of the previous scan cycle.

Question 15

Consider a PLC program utilizing a standard `TON` (Timer On-Delay) function block. The timer is configured with a preset time (`PT`) of $1000$ milliseconds. The timer\'s `IN` input is driven by a momentary contact that is activated for precisely $500$ milliseconds before being deactivated. What will be the state of the `Q` output and the `ET` (Elapsed Time) output of the `TON` function block immediately after the $500$ millisecond activation period concludes?

Accepted Answer

Q = FALSE, ET = $500$ ms

Question 16

Consider a PLC program utilizing the IEC 61131-3 standard. A function block instance of `R_TRIG` is declared, and its input variable `CLK` is connected to a digital input that is initially TRUE, then held TRUE for several PLC scan cycles, and subsequently transitions to FALSE. What will be the state of the `R_TRIG` function block\'s output variable `Q` during the scan cycle immediately following the transition of `CLK` from TRUE to FALSE?

Accepted Answer

FALSE

Question 17

Consider a scenario within a process control application where a `TON` function block is configured with a preset time (`PT`) of 500 milliseconds. The timer\'s input (`IN`) is connected to a momentary push button that is pressed and released within a single PLC scan cycle, lasting only 20 milliseconds. Assuming the PLC\'s scan time is consistently 10 milliseconds, what will be the state of the timer\'s output (`Q`) and its elapsed time (`ET`) at the end of the scan cycle immediately following the button release?

Accepted Answer

Q = FALSE, ET = 0

Question 18

Consider a PLC program utilizing the `R_TRIG` function block from IEC 61131-3. An instance of this function block, named `EdgeDetector`, has its input `CLK` permanently connected to a digital input that is always in the TRUE state. What will be the state of the `Q` output of the `EdgeDetector` instance during the PLC\'s continuous operation?

Accepted Answer

FALSE

Question 19

Consider a scenario within a machine control system where a `TON` function block is utilized to activate a cooling fan. The `IN` input of the `TON` is connected to a proximity sensor detecting the presence of a workpiece, and the `PT` (Preset Time) is configured to 2 seconds. The cooling fan should only operate if the workpiece remains in position for the full 2 seconds. If the workpiece is removed before the 2 seconds elapse, the cooling fan should immediately stop, and the timer should reset. What is the state of the `Q` output and the `ET` (Elapsed Time) value of the `TON` function block immediately after the `IN` input transitions from TRUE to FALSE, assuming the `IN` input was TRUE for only 1.5 seconds before this transition?

Accepted Answer

Q is FALSE, ET is 0

Question 20

Consider a PLC program where a variable `WORD_VAR` of type `WORD` is initialized to the hexadecimal value `16#00FF`. Subsequently, the following two operations are executed sequentially within the same PLC scan cycle: first, `SET(WORD_VAR.5)` is called, and immediately after, `RESET(WORD_VAR.2)` is called. What will be the final hexadecimal value of `WORD_VAR` after these operations?

Accepted Answer

16#00BB

Question 21

Consider a scenario where a function block named `MotorControl` is defined with internal state variables for motor speed and direction. This `MotorControl` function block is called within a cyclic task\'s main program organization unit (POU). If the `MotorControl` function block is declared using the `VAR` keyword directly within the POU\'s local variable declaration section, what will be the state of its internal variables at the beginning of each subsequent execution cycle of that POU?

Accepted Answer

The internal variables will be re-initialized to their declared default values.

Question 22

Consider a system where a function block instance, `FB_ProcessData`, is declared globally and invoked within the main cyclic task. This function block contains an internal `TON` (Timer On Delay) instance named `TON_Delay` with a preset time of $5$ seconds. The `TON_Delay` is activated by an input signal `I_Start`. If `I_Start` is asserted for $3$ seconds, then de-asserted for $2$ seconds, and subsequently asserted again for $2$ seconds, what will be the state of the `Q_Ready` output of `FB_ProcessData` at the end of this sequence?

Accepted Answer

FALSE

Question 23

Consider a PLC program where a Function Block instance, `myMotor`, of type `MOTOR_CONTROL` is declared. The `MOTOR_CONTROL` FB has an internal state variable `_state` and inputs `START` (BOOL) and outputs `RUNNING` (BOOL). The FB\'s logic is as follows: if `START` is TRUE, `_state` is set to `RUNNING`; otherwise, `_state` is set to `STOPPED`. The `RUNNING` output is directly assigned the value of `_state`. Within a single PLC scan cycle, the `myMotor` instance is called twice: first with `START` set to TRUE, and immediately thereafter with `START` set to FALSE. What will be the final value of the `RUNNING` output of `myMotor` at the end of this scan cycle?

Accepted Answer

FALSE

Question 24

Consider a scenario involving a `TON` function block programmed in Structured Text. The `IN` input of the `TON` block is pulsed TRUE for 2 seconds, and then immediately transitions to FALSE. The preset time (`PT`) for this `TON` block is configured to 5 seconds. What will be the value of the elapsed time (`ET`) output of the `TON` function block immediately after the `IN` input transitions from TRUE to FALSE?

Accepted Answer

$T\#0s$

Question 25

Consider a Function Block `MOTOR_CONTROL` defined with an internal variable `_MotorState` of type `INT` within its `VAR` section. This FB is instantiated in an ST program. Which of the following statements accurately describes the ability to modify `_MotorState` from the calling ST program?

Accepted Answer

Direct modification of `_MotorState` from the calling ST program is not permitted by the standard, as it is an internal variable of the Function Block.

Question 26

Consider a Programmable Logic Controller (PLC) executing a program that utilizes the standard IEC 61131-3 `TON` (Timer On Delay) function block. The `PT` (Preset Time) parameter of this timer is configured to 500 milliseconds. The `IN` (Input) of the `TON` function block is asserted TRUE for a duration of 300 milliseconds, after which it is immediately de-asserted to FALSE. What will be the value of the `ET` (Elapsed Time) parameter of the `TON` function block immediately after the `IN` input transitions from TRUE to FALSE?

Accepted Answer

0 milliseconds

Question 27

Consider a scenario involving a conveyor belt control system where a `TON` function block is utilized to activate a warning siren if a specific component fails to arrive at a station within a designated timeframe. The `IN` input of the `TON` is connected to a proximity sensor that briefly detects the component. The `PT` (Preset Time) is configured to 100 milliseconds. The proximity sensor, due to intermittent signal issues, generates a pulse of 20 milliseconds every 50 milliseconds. What will be the state of the `Q` output and the `ET` (Elapsed Time) value of the `TON` function block after several such pulses have occurred?

Accepted Answer

The `Q` output remains FALSE, and the `ET` value continuously resets to 0 ms.

Question 28

Consider a PLC program utilizing the `TON` (Timer On Delay) function block from IEC 61131-3. The function block instance, named `MyTimer`, is configured with a preset time (`PT`) of $500$ milliseconds. The `IN` input of `MyTimer` is driven by a digital input that is pulsed for a duration of $300$ milliseconds. What will be the state of the `Q` output and the elapsed time (`ET`) of `MyTimer` immediately after the $300$ millisecond pulse on the `IN` input concludes?

Accepted Answer

Q = FALSE, ET = 0 ms

Question 29

Consider a PLC program utilizing the `R_TRIG` function block from the IEC 61131-3 standard. The `R_TRIG` instance, named `EdgeDetector`, has its `CLK` input connected to a physical input that transitions from FALSE to TRUE at the beginning of scan cycle 1 and remains TRUE through scan cycle 2. What will be the state of the `Q` output of the `EdgeDetector` instance during scan cycle 2?

Accepted Answer

FALSE

IEC 61131-3:2013 - PLC Programming Professional Free Practice Test — 29 Questions

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