Operator input screens in PLC Programming - Step-by-Step Execution

Concept Flow - Operator input screens

Start

↓

Display Input Screen

↓

Wait for Operator Input

↓

Validate Input

↓

Process Input

↓

Update System

↓

End or Repeat

The operator input screen shows prompts, waits for input, checks it, processes valid input, or shows errors and repeats.

Execution Sample

PLC Programming

DISPLAY 'Enter Speed:'
WAIT_FOR_INPUT speed
IF speed < 0 OR speed > 100 THEN
  DISPLAY 'Error: Speed out of range'
  RETURN TO INPUT
ELSE
  SET motor_speed = speed
END IF

This code shows a prompt, waits for speed input, checks if it's between 0 and 100, shows error if not, else sets motor speed.

Execution Table

Step	Action	Input/Condition	Result	Next Step
1	Display prompt	N/A	'Enter Speed:' shown	Wait for input
2	Wait for input	Operator enters 50	Input received: 50	Validate input
3	Validate input	50 >= 0 and <= 100	Valid input	Set motor_speed = 50
4	Set motor_speed	motor_speed = 50	Motor speed updated	End or repeat
5	Display prompt	N/A	'Enter Speed:' shown	Wait for input
6	Wait for input	Operator enters 150	Input received: 150	Validate input
7	Validate input	150 > 100	Invalid input	Display error
8	Display error	'Error: Speed out of range'	Error shown	Return to input screen
9	Return to input screen	N/A	Back to prompt	Wait for input

💡 Execution stops when valid input is processed or operator cancels input.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 6	After Step 7	After Step 8
speed	undefined	50	50	50	150	150	150
motor_speed	undefined	undefined	undefined	50	50	50	50

Key Moments - 3 Insights

Why does the screen show an error message after entering 150?

Because at step 7 the input 150 is outside the allowed range (0-100), so the program shows an error and returns to input (see execution_table rows 7-9).

What happens to motor_speed if the operator enters a valid speed?

At step 4, motor_speed is updated to the valid input value (50), as shown in variable_tracker and execution_table row 4.

Does the program stop after one input attempt?

No, if input is invalid, the program loops back to show the input screen again (execution_table rows 8-9), allowing repeated attempts.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of motor_speed after step 4?

A 50

B undefined

C 150

D 0

💡 Hint

Check the variable_tracker for motor_speed after step 4.

At which step does the program detect invalid input?

A Step 3

B Step 2

C Step 7

D Step 9

💡 Hint

Look at the execution_table row where input is checked and error is shown.

If the operator enters 75 instead of 150, what changes in the execution table?

A The program shows an error anyway

B The error message steps are skipped

C The motor_speed remains undefined

D The program exits immediately

💡 Hint

Refer to how valid input is handled in execution_table rows 2-4.

Concept Snapshot

Operator input screens show prompts to the user.
They wait for input, then check if input is valid.
If valid, input updates system variables.
If invalid, show error and ask again.
This loop continues until valid input is entered.

Full Transcript

Operator input screens in PLC programming display prompts to the operator and wait for input. The input is then validated against allowed ranges or conditions. If the input is valid, it updates system variables such as motor speed. If invalid, an error message is shown and the input screen is displayed again for correction. This process repeats until the operator enters valid data or cancels. The execution table traces these steps showing prompt display, input reception, validation, error handling, and variable updates. Variables like speed and motor_speed change values as input is processed. Key moments include understanding why errors appear, how variables update, and the looping nature of input screens. The visual quiz tests understanding of variable states and flow control in the input process.

Practice

1. What is the main purpose of an operator input screen in PLC programming?

easy

A. To allow users to enter data and control the machine

B. To display error logs only

C. To run the PLC program automatically

D. To store historical data for analysis

Solution

1. Step 1: Understand the role of operator input screens

   Operator input screens are designed for users to interact with the machine by entering commands or data. The main purpose is to allow users to enter data and control the machine.

2. Final Answer:

   To allow users to enter data and control the machine -> Option A

3. Quick Check:

   Operator input screens = user control [OK]

Hint: Think: Who uses the screen and why? [OK]

Common Mistakes:

• Confusing input screens with data storage
• Thinking input screens only display errors
• Assuming input screens run the program automatically

2. Which of the following is the correct syntax to define a numeric input field named SpeedInput in a PLC operator screen?

easy

A. Define SpeedInput as NumericInput

B. NumericInput SpeedInput

C. InputField SpeedInput Numeric

D. SpeedInput = NumericInput()

Solution

1. Step 1: Review common syntax for input fields

   In many PLC screen definitions, input fields are declared with the keyword InputField followed by the name and type, such as InputField SpeedInput Numeric. Other forms use incorrect or unsupported syntax.

2. Final Answer:

   InputField SpeedInput Numeric -> Option C

3. Quick Check:

   InputField + name + type = correct syntax [OK]

Hint: Look for keyword 'InputField' with name and type order [OK]

Common Mistakes:

• Using assignment syntax instead of declaration
• Omitting the InputField keyword
• Mixing up order of name and type

3. Given this snippet of a PLC operator screen script:

InputField Temperature Numeric
Button Submit
DisplayMessage "Enter temperature and press Submit"

What will the operator see when the screen loads?

medium

A. An error because DisplayMessage is not valid

B. Only a Submit button and a message

C. A numeric input box labeled Submit and a Temperature button

D. A numeric input box labeled Temperature, a Submit button, and a message prompting input

Solution

1. Step 1: Analyze each line of the script

   The script defines a numeric input field named Temperature, a button named Submit, and a message "Enter temperature and press Submit". The operator will see the input box for Temperature, the Submit button, and the message prompting input.

2. Final Answer:

   A numeric input box labeled Temperature, a Submit button, and a message prompting input -> Option D

3. Quick Check:

   InputField + Button + DisplayMessage = all visible [OK]

Hint: Match script lines to screen elements shown [OK]

Common Mistakes:

• Ignoring the DisplayMessage line
• Confusing button and input field names
• Assuming syntax errors without checking

4. Identify the error in this operator input screen code snippet:

InputField Speed Numeric
Button Start
DisplayMessage "Set speed and press Start"
InputField Speed Text

medium

A. Speed input field is declared twice with different types

B. Button Start is missing parentheses

C. DisplayMessage should be inside quotes

D. No error, code is correct

Solution

1. Step 1: Review declarations of input fields

   The code declares Speed as a Numeric input field first, then again as a Text input field. Declaring the same input field twice with different types causes conflict and confusion in the screen design.

2. Final Answer:

   Speed input field is declared twice with different types -> Option A

3. Quick Check:

   Duplicate input fields with different types = error [OK]

Hint: Check for repeated names with different types [OK]

Common Mistakes:

• Thinking button syntax is wrong
• Ignoring duplicate input field declarations
• Assuming DisplayMessage syntax error

5. You want to create an operator input screen that asks for two numeric values, Length and Width, and calculates the area when the user presses a Calculate button. Which script snippet correctly sets up the input fields and button to achieve this?

hard

A. InputField Length Numeric
InputField Width Numeric
Button Calculate
OnPress CalculateArea(Length, Width)

B. InputField Length Numeric
InputField Width Numeric
Button Calculate
OnButton Calculate CalculateArea(Length, Width)

C. InputField Length Text
InputField Width Text
Button Calculate
OnButton Calculate CalculateArea(Length, Width)

D. InputField Length Numeric
InputField Width Numeric
Button Calculate
OnClick CalculateArea(Length, Width)

Solution

1. Step 1: Verify input fields and button event syntax

   Length and Width must be Numeric inputs for area calculation; Text inputs are incorrect. The button event requires the standard syntax OnButton Calculate CalculateArea(Length, Width). Event keywords like OnPress or OnClick are not standard.

2. Final Answer:

   InputField Length Numeric InputField Width Numeric Button Calculate OnButton Calculate CalculateArea(Length, Width) -> Option B

3. Quick Check:

   Numeric inputs + OnButton event = correct setup [OK]

Hint: Use numeric inputs and OnButton event for calculations [OK]

Common Mistakes:

• Using Text input for numeric values
• Using wrong event keywords like OnPress or OnClick
• Omitting event handler for button