PLC hardware architecture (CPU, I/O, power supply) in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

The CPU (Central Processing Unit) is the brain of the PLC. It processes the input signals, executes the control program, and sends commands to the output devices.

beginner

What types of signals do PLC input modules handle?

PLC input modules handle signals from sensors or switches. These can be digital (on/off) or analog (varying voltage or current).

beginner

Why is the power supply important in a PLC system?

The power supply provides the necessary electrical energy to the CPU, input/output modules, and other parts of the PLC to operate correctly.

beginner

How do output modules in a PLC work?

Output modules receive commands from the CPU and control actuators like motors, lights, or valves by sending electrical signals.

beginner

What are the main components of a PLC hardware architecture?

The main components are the CPU (processing unit), input modules (to receive signals), output modules (to control devices), and the power supply (to provide energy).

Which PLC component processes the control program?

A Output module

B Input module

C CPU

D Power supply

The CPU executes the control program and manages the PLC operations.

What type of signals do input modules receive?

A Signals from sensors

B Signals from actuators

C Power signals

D Control commands

Input modules receive signals from sensors or switches to inform the PLC about the system status.

What is the main purpose of the power supply in a PLC?

A To provide electrical energy

B To send output signals

C To receive input signals

D To process data

The power supply provides the electrical energy needed for the PLC components to work.

Which module sends commands to control devices like motors?

A CPU

B Power supply

C Input module

D Output module

Output modules send electrical signals to actuators such as motors or lights.

Which of these is NOT a main component of PLC hardware architecture?

A Input module

B Display screen

C CPU

D Power supply

Display screens are not a core hardware component of PLC architecture.

Explain the roles of CPU, input/output modules, and power supply in a PLC system.

Think about how each part helps the PLC work like a small brain controlling machines.

Describe how input and output modules interact with the CPU in a PLC.

Imagine the PLC as a messenger between sensors and machines.

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