Troubleshooting PLC systems in PLC Programming - Step-by-Step Execution

Concept Flow - Troubleshooting PLC systems

Start Troubleshooting

↓

Check Power Supply

↓

Inspect Input Signals

↓

Verify PLC Program

↓

Test Output Devices

↓

Use Diagnostic Tools

↓

Fix Identified Issues

↓

Test System Operation

↓

End Troubleshooting

This flow shows the step-by-step process to find and fix problems in a PLC system, starting from power checks to testing the system after fixes.

Execution Sample

PLC Programming

IF input_sensor = ON THEN
  output_motor := ON;
ELSE
  output_motor := OFF;
END_IF;

This simple PLC logic turns a motor on if the input sensor is on, otherwise it turns the motor off.

Execution Table

Step	Input Sensor	Condition (input_sensor=ON?)	Output Motor	Action Taken
1	OFF	False	OFF	Motor remains off because sensor is off
2	ON	True	ON	Motor turns on because sensor is on
3	OFF	False	OFF	Motor turns off because sensor is off

💡 Troubleshooting ends after verifying output motor matches input sensor state

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
input_sensor	OFF	OFF	ON	OFF
output_motor	OFF	OFF	ON	OFF

Key Moments - 3 Insights

Why does the motor stay off when the input sensor is off?

Because the condition input_sensor=ON is false (see execution_table step 1), so the motor output is set to OFF.

What should you check if the motor does not turn on when the sensor is ON?

Check the wiring and input signal to confirm the sensor is actually ON, and verify the PLC program logic matches the expected condition (see execution_table step 2).

Why is it important to test the system after fixing issues?

Testing confirms that the fixes worked and the PLC outputs respond correctly to inputs, as shown in the final step of the execution_table.

Visual Quiz - 3 Questions

Look at the execution table, what is the output_motor value at step 2?

A ON

B OFF

C Undefined

D Error

💡 Hint

Check the 'Output Motor' column in execution_table row for step 2

At which step does the condition input_sensor=ON evaluate to false?

A Step 1

B Step 2

C Step 3

D None

💡 Hint

Look at the 'Condition' column in execution_table for steps where input_sensor is OFF

If the input_sensor stays ON but output_motor remains OFF, what should you check first?

A Power supply to PLC

B PLC program logic

C Output device wiring

D Input sensor wiring

💡 Hint

Refer to key_moments about verifying PLC program logic when output does not match input

Concept Snapshot

Troubleshooting PLC systems:
1. Check power supply and wiring.
2. Verify input signals and sensors.
3. Review PLC program logic.
4. Test output devices.
5. Use diagnostic tools.
6. Fix issues and retest system.

Full Transcript

Troubleshooting PLC systems involves a step-by-step process starting with checking the power supply, then inspecting input signals, verifying the PLC program, testing output devices, using diagnostic tools, fixing identified issues, and finally testing the system operation to ensure everything works correctly. The example PLC code shows a simple condition where a motor output turns on if an input sensor is on. The execution table traces how the motor output changes based on the sensor input. Key moments clarify why outputs change or stay the same and what to check if outputs do not respond as expected. The visual quiz tests understanding of the execution steps and troubleshooting priorities.

Practice

1. What is the first step when troubleshooting a PLC system that is not starting?

easy

A. Reset the factory settings without checking

B. Rewrite the entire PLC program

C. Replace all wiring immediately

D. Check if the PLC has power and is turned on

Solution

1. Step 1: Verify power supply to the PLC

   Without power, the PLC cannot operate, so checking power is the first logical step.

2. Step 2: Confirm PLC is turned on and indicators are normal

   Indicators like LEDs show if the PLC is active or has faults.

3. Final Answer:

   Check if the PLC has power and is turned on -> Option D

4. Quick Check:

   Power check = first troubleshooting step [OK]

Hint: Always check power first before anything else [OK]

Common Mistakes:

• Jumping to program changes without checking power
• Replacing wiring without diagnosis
• Resetting without understanding the problem

2. Which of the following is the correct syntax to read an input named I0.0 in a PLC ladder logic program?

easy

A. INPUT(I0.0)

B. READ I0.0

C. LD I0.0

D. GET I0.0

Solution

1. Step 1: Identify standard ladder logic instruction for input

   LD (Load) is the common instruction to read an input bit in ladder logic.

2. Step 2: Compare options with standard syntax

   Only 'LD I0.0' matches typical ladder logic syntax for reading input.

3. Final Answer:

   LD I0.0 -> Option C

4. Quick Check:

   LD = load input instruction [OK]

Hint: Use LD to load inputs in ladder logic [OK]

Common Mistakes:

• Using non-standard commands like READ or GET
• Adding parentheses incorrectly
• Confusing input reading with output commands

3. Given this PLC ladder logic snippet:

LD I0.1
AND I0.2
OUT Q0.0

What will be the output Q0.0 if I0.1 is ON and I0.2 is OFF?

medium

A. Q0.0 will be ON

B. Q0.0 will be OFF

C. Q0.0 will toggle ON and OFF rapidly

D. The program will cause an error

Solution

1. Step 1: Evaluate the AND condition with inputs

   AND requires both I0.1 and I0.2 to be ON. Here, I0.1 is ON but I0.2 is OFF.

2. Step 2: Determine output based on AND result

   Since one input is OFF, the AND condition is false, so Q0.0 will be OFF.

3. Final Answer:

   Q0.0 will be OFF -> Option B

4. Quick Check:

   AND false = output OFF [OK]

Hint: AND needs all inputs ON for output ON [OK]

Common Mistakes:

• Assuming output ON if any input is ON
• Confusing AND with OR logic
• Expecting errors from simple logic

4. A PLC program is not turning on an output Q0.1 even though the input I0.3 is ON. The code snippet is:

LD I0.3
OUT Q0.1

What is the most likely error?

medium

A. The output address Q0.1 is incorrect or not configured

B. The input I0.3 cannot be ON in PLC logic

C. The LD instruction is invalid syntax

D. The PLC program must use AND before OUT

Solution

1. Step 1: Check input and output addresses

   Input I0.3 is ON, so the problem is likely with output Q0.1 address or configuration.

2. Step 2: Validate output wiring and PLC configuration

   If Q0.1 is not configured or wired correctly, output won't activate despite correct logic.

3. Final Answer:

   The output address Q0.1 is incorrect or not configured -> Option A

4. Quick Check:

   Output config error = no output ON [OK]

Hint: Check output address and wiring if output stays OFF [OK]

Common Mistakes:

• Assuming input cannot be ON
• Thinking LD is wrong syntax
• Believing AND is always needed before OUT

5. You have a PLC controlling a conveyor belt. The belt should run only if both sensors I0.0 and I0.1 detect objects. The current program uses:

LD I0.0
OR I0.1
OUT Q0.0

What change should you make to fix the logic so the belt runs only when both sensors detect objects?

hard

A. Change OR to AND in the program

B. Add a NOT instruction before I0.1

C. Replace OUT with IN instruction

D. Remove I0.0 from the program

Solution

1. Step 1: Understand the current logic

   OR means the belt runs if either sensor detects an object, but requirement is both sensors must detect.

2. Step 2: Replace OR with AND to require both sensors ON

   AND ensures output Q0.0 is ON only when both I0.0 and I0.1 are ON.

3. Final Answer:

   Change OR to AND in the program -> Option A

4. Quick Check:

   AND = both inputs ON for output ON [OK]

Hint: Use AND to require all conditions true [OK]

Common Mistakes:

• Using OR when AND is needed
• Adding NOT incorrectly
• Confusing OUT with input instructions