Trend charts for process variables in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a simple trend data logging loop

Consider a PLC program snippet that logs a process variable every second into an array of 5 elements. What will be the content of the array after 5 seconds if the variable increments by 2 each second starting from 0?

PLC Programming

VAR
  TrendArray: ARRAY[1..5] OF INT := (0,0,0,0,0);
  ProcessVar: INT := 0;
  i: INT := 1;
END_VAR

// Each second:
TrendArray[i] := ProcessVar;
ProcessVar := ProcessVar + 2;
i := i + 1;

A [0, 2, 4, 6, 8]

B [0, 0, 0, 0, 0]

C [2, 4, 6, 8, 10]

D [1, 3, 5, 7, 9]

💡 Hint

Think about how the variable increments and when the array is updated.

The array stores the value of ProcessVar before it increments. Starting at 0, it stores 0, then 2, then 4, 6, and finally 8 after 5 seconds.

🧠 Conceptual

intermediate

Understanding sampling rate impact on trend charts

Why is it important to choose an appropriate sampling rate when logging process variables for trend charts in PLCs?

A Because too slow sampling may miss important changes, and too fast sampling may overload memory.

B Because sampling rate does not affect trend chart accuracy.

C Because faster sampling always produces better trend charts regardless of memory.

D Because sampling rate only affects the display color of the trend chart.

💡 Hint

Think about what happens if you record too few or too many data points.

Sampling too slowly can miss quick changes in the process variable, making the trend inaccurate. Sampling too fast can fill memory quickly and slow down the system.

🔧 Debug

advanced

Identify the error in trend data array update

Given this PLC code snippet for updating a trend array, what error will occur? VAR TrendArray: ARRAY[1..5] OF INT := (0,0,0,0,0); ProcessVar: INT := 0; i: INT := 1; END_VAR TrendArray[i] := ProcessVar; i := i + 1; IF i > 5 THEN i := 1; END_IF;

A Syntax error: Missing semicolon after IF statement.

B Runtime error: Index out of bounds when i becomes 6.

C No error; the array updates correctly in a circular manner.

D Logic error: ProcessVar is never incremented.

💡 Hint

Check how the index variable i is reset after reaching the array size.

The index i increments each cycle and resets to 1 after exceeding 5, so the array updates in a circular buffer style without errors.

🚀 Application

advanced

Creating a trend chart with time stamps

You want to log a process variable and its timestamp every minute for 10 minutes in a PLC. Which data structure best suits this requirement?

A A string variable concatenating all values and timestamps.

B A single array of INT with size 20, alternating values and timestamps.

C A single INT variable updated every minute.

D Two arrays: one for process variable values and one for timestamps, both size 10.

💡 Hint

Think about how to keep values and their corresponding times linked clearly.

Using two parallel arrays keeps values and timestamps aligned by index, making it easy to plot trend charts with time.

🧠 Conceptual

expert

Effect of data smoothing on trend chart interpretation

How does applying a moving average smoothing algorithm to process variable data affect the trend chart interpretation?

A It changes the time scale of the trend chart.

B It reduces noise and makes trends clearer but may hide sudden changes.

C It increases noise and makes the chart harder to read.

D It removes all data points except the maximum value.

💡 Hint

Consider what smoothing does to rapid fluctuations in data.

Moving average smooths out short-term fluctuations, making overall trends easier to see but can mask quick spikes or drops.

Practice

1. What is the main purpose of a trend chart in PLC programming for process variables?

easy

A. To show how process variables change over time

B. To control the speed of the PLC processor

C. To reset the PLC memory automatically

D. To display error messages only

Solution

1. Step 1: Understand the role of trend charts

   Trend charts are used to visualize data changes over time, especially for process variables like temperature or pressure.

2. Step 2: Eliminate unrelated options

   Options about controlling processor speed, resetting memory, or showing errors do not relate to trend charts.

3. Final Answer:

   To show how process variables change over time -> Option A

4. Quick Check:

   Trend charts = show variable changes over time [OK]

Hint: Trend charts visualize data changes over time [OK]

Common Mistakes:

• Confusing trend charts with control functions
• Thinking trend charts reset PLC memory
• Assuming trend charts only show errors

2. Which of the following is the correct way to declare an array to store 10 samples of a process variable in a PLC program?

easy

A. ARRAY samples[10] REAL;

B. samples = [10] REAL;

C. VAR samples: REAL[10]; END_VAR

D. VAR samples: ARRAY[1..10] OF REAL; END_VAR

Solution

1. Step 1: Recognize PLC array declaration syntax

   In many PLC languages, arrays are declared with VAR, specifying index range and type, e.g., ARRAY[1..10] OF REAL.

2. Step 2: Check each option

   VAR samples: ARRAY[1..10] OF REAL; END_VAR matches correct syntax. Options A, B, and D use incorrect or invalid syntax for PLC arrays.

3. Final Answer:

   VAR samples: ARRAY[1..10] OF REAL; END_VAR -> Option D

4. Quick Check:

   Correct PLC array syntax = VAR samples: ARRAY[1..10] OF REAL; END_VAR [OK]

Hint: Use VAR and ARRAY with index range for PLC arrays [OK]

Common Mistakes:

• Using incorrect brackets or missing VAR keyword
• Confusing array syntax with other languages
• Not specifying index range properly

3. Given the following PLC code snippet that samples a temperature every cycle and stores it in an array of size 5, what will be the content of the array after 7 cycles?

VAR samples: ARRAY[1..5] OF REAL; index: INT := 1; temp: REAL; END_VAR

// On each cycle:
temp := ReadTemperature();
samples[index] := temp;
index := index + 1;
IF index > 5 THEN
  index := 1;
END_IF

medium

A. The array is empty because index resets too early

B. The array contains the first 5 temperature samples from cycles 1 to 5

C. The array contains the last 5 temperature samples from cycles 3 to 7

D. The array contains only the temperature from cycle 7 repeated

Solution

1. Step 1: Understand the circular buffer logic

   The index increments each cycle and resets to 1 after 5, so the array stores the last 5 samples in a loop.

2. Step 2: Analyze after 7 cycles

   After 7 cycles, index has wrapped around once. The array holds samples from cycles 3 to 7, overwriting the first two samples.

3. Final Answer:

   The array contains the last 5 temperature samples from cycles 3 to 7 -> Option C

4. Quick Check:

   Circular buffer stores last 5 samples = The array contains the last 5 temperature samples from cycles 3 to 7 [OK]

Hint: Index resets after max size to overwrite oldest samples [OK]

Common Mistakes:

• Thinking array holds first 5 samples only
• Assuming array is empty due to index reset
• Believing array repeats last sample only

4. Identify the error in this PLC code snippet intended to sample a pressure value every 10 cycles and store it in an array:

VAR samples: ARRAY[1..5] OF REAL; counter: INT := 0; index: INT := 1; pressure: REAL; END_VAR

counter := counter + 1;
IF counter = 10 THEN
  samples[index] := ReadPressure();
  index := index + 1;
  counter := 0;
  IF index > 5 THEN
    index := 1;
  END_IF
END_IF

medium

A. The counter increments every cycle but sampling happens only when counter equals 10

B. The index reset condition should be inside the first IF block

C. The counter should reset after index increments, not before

D. The index reset condition is outside the sampling IF block, causing incorrect indexing

Solution

1. Step 1: Review index reset placement

   The index reset IF was outside the sampling IF, causing it to run every cycle and reset index prematurely.

2. Step 2: Correct placement

   Moving the index reset condition inside the sampling IF block ensures index resets only after a new sample is stored, preventing incorrect indexing.

3. Final Answer:

   The index reset condition should be inside the first IF block -> Option B

4. Quick Check:

   Index reset must be inside sampling block [OK]

Hint: Keep index reset inside sampling condition to avoid errors [OK]

Common Mistakes:

• Placing index reset outside sampling condition
• Resetting counter at wrong time
• Misunderstanding counter increment logic

5. You want to create a trend chart that samples a flow rate every 5 seconds and stores the last 12 samples. Which approach correctly implements this in a PLC program?

hard

A. Use a timer that triggers every 5 seconds to read flow rate, store it in a 12-element array, and use a circular index to overwrite oldest samples.

B. Sample flow rate every cycle without timing, store in a 12-element array, reset index after 12 cycles.

C. Use a timer to sample every 12 seconds, store in a 5-element array, and reset index after 5 samples.

D. Sample flow rate only once at start, store in a 12-element array, then never update.

Solution

1. Step 1: Match sampling frequency and storage size

   The requirement is to sample every 5 seconds and keep last 12 samples, so timer interval and array size must match.

2. Step 2: Use circular buffer logic

   Using a timer to trigger sampling every 5 seconds and a circular index to overwrite oldest samples in a 12-element array fits the need.

3. Final Answer:

   Use a timer that triggers every 5 seconds to read flow rate, store it in a 12-element array, and use a circular index to overwrite oldest samples. -> Option A

4. Quick Check:

   Timer + circular buffer = correct trend sampling [OK]

Hint: Timer triggers sampling; circular buffer stores last samples [OK]

Common Mistakes:

• Sampling every cycle without timing control
• Using wrong timer interval or array size
• Not overwriting oldest samples properly