PLC hardware architecture (CPU, I/O, power supply) in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Identify the main function of the PLC CPU

What is the primary role of the CPU in a PLC system?

A To execute the control program and process input/output signals

B To supply power to all PLC modules

C To physically connect sensors and actuators

D To provide network communication only

💡 Hint

Think about what the brain of a computer does.

The CPU in a PLC executes the control program and processes signals from inputs and outputs, acting as the brain of the system.

💻 Command Output

intermediate

Determine the number of digital inputs from a PLC I/O module

A PLC digital input module has 16 channels. If all inputs are off, what is the hexadecimal value of the input status register?

PLC Programming

Input status register = 0x0000

A 0x00

B 0xFFFF

C 0xFF

D 0x0F

💡 Hint

All inputs off means all bits are zero.

With 16 digital inputs all off, the input status register bits are all zero, so the value is 0x0000 (for 16 bits).

📝 Syntax

advanced

Identify the correct power supply voltage for a PLC system

Which of the following is a typical power supply voltage for a standard PLC system?

A 220 V DC

B 120 V AC

C 5 V DC

D 24 V DC

💡 Hint

PLC power supplies usually use low voltage DC for safety.

Most PLCs use 24 V DC power supplies as standard for safety and compatibility with industrial sensors and actuators.

🔧 Debug

advanced

Troubleshoot a PLC I/O module not responding

A PLC digital output module is not activating connected devices. Which hardware issue is most likely the cause?

A Faulty power supply to the output module

B CPU program logic error

C Input module failure

D Incorrect network configuration

💡 Hint

If outputs do not activate physically, check power first.

If the output module has no power, it cannot activate devices even if the CPU sends commands.

🚀 Application

expert

Calculate total power consumption of a PLC system

A PLC CPU consumes 5 W, each digital input module consumes 2 W, and each digital output module consumes 3 W. If the system has 1 CPU, 3 input modules, and 2 output modules, what is the total power consumption?

A 16 W

B 15 W

C 17 W

D 18 W

💡 Hint

Add power of all components: CPU + inputs + outputs.

Total power = CPU (5 W) + 3 inputs × 2 W + 2 outputs × 3 W = 5 + 6 + 6 = 17 W.

Practice

1. Which part of a PLC acts as the brain, processing all instructions?

easy

A. Input module

B. Power supply

C. CPU

D. Output module

Solution

1. Step 1: Understand the role of CPU in PLC

   The CPU processes instructions and controls the PLC operation.

2. Step 2: Compare with other parts

   Power supply provides energy, input modules receive signals, output modules send commands.

3. Final Answer:

   CPU -> Option C

4. Quick Check:

   CPU = Brain of PLC [OK]

Hint: CPU is the brain that thinks and controls [OK]

Common Mistakes:

• Confusing power supply with CPU
• Thinking input module processes data
• Mixing output module with CPU

2. Which of the following is the correct symbol for the power supply in a PLC diagram?

easy

A. A box labeled 'PS' or 'Power Supply'

B. A circle labeled 'I/O'

C. An arrow pointing to output devices

D. A rectangle labeled 'CPU'

Solution

1. Step 1: Identify common PLC diagram symbols

   Power supply is usually shown as a box labeled 'PS' or 'Power Supply'.

2. Step 2: Eliminate other options

   CPU is labeled 'CPU', I/O is not a circle, arrows show signal flow not power supply.

3. Final Answer:

   A box labeled 'PS' or 'Power Supply' -> Option A

4. Quick Check:

   Power supply symbol = Box labeled PS [OK]

Hint: Power supply is labeled 'PS' in diagrams [OK]

Common Mistakes:

• Choosing CPU box as power supply
• Confusing arrows with power supply
• Thinking I/O is power supply symbol

3. Given a PLC with 8 input sensors connected to the input module and 6 output devices connected to the output module, what is the total number of I/O points?

medium

A. 48

B. 8

C. 6

D. 14

Solution

1. Step 1: Understand I/O points

   I/O points are the sum of input points and output points connected to the PLC.

2. Step 2: Calculate total I/O points

   8 inputs + 6 outputs = 14 total I/O points.

3. Final Answer:

   14 -> Option D

4. Quick Check:

   8 + 6 = 14 I/O points [OK]

Hint: Add input and output counts for total I/O [OK]

Common Mistakes:

• Counting only inputs
• Counting only outputs
• Multiplying inputs by outputs

4. A PLC system is not powering on. Which of the following is the most likely hardware issue?

medium

A. Faulty CPU

B. Disconnected power supply

C. Broken output module

D. Input module error

Solution

1. Step 1: Analyze symptoms

   PLC not powering on means no power is reaching the system.

2. Step 2: Identify hardware responsible for power

   The power supply provides energy; if disconnected or faulty, PLC won't start.

3. Final Answer:

   Disconnected power supply -> Option B

4. Quick Check:

   No power = Power supply issue [OK]

Hint: No power means check power supply first [OK]

Common Mistakes:

• Blaming CPU when no power
• Assuming output module stops power
• Ignoring power supply connection

5. You need to expand a PLC system to control 12 new sensors and 10 new actuators. Which hardware components must you add to the existing PLC setup?

hard

A. Add input and output modules

B. Add more CPU units

C. Add extra power supplies only

D. Replace the existing CPU with a bigger one

Solution

1. Step 1: Identify hardware for sensors and actuators

   Sensors connect to input modules; actuators connect to output modules.

2. Step 2: Determine what to add

   To handle more devices, add input modules for sensors and output modules for actuators.

3. Step 3: Consider CPU and power supply

   CPU usually handles more modules without replacement; power supply may need upgrade but not always.

4. Final Answer:

   Add input and output modules -> Option A

5. Quick Check:

   More devices = Add I/O modules [OK]

Hint: Add I/O modules to handle more devices [OK]

Common Mistakes:

• Thinking CPU must be replaced
• Adding only power supplies
• Adding CPU units instead of modules