PLC hardware architecture (CPU, I/O, power supply) in PLC Programming - Step-by-Step Execution

Concept Flow - PLC hardware architecture (CPU, I/O, power supply)

Power Supply

↓

CPU (Central Processing Unit)

↓

Input Modules

↓

Field Devices (Sensors/Actuators)

Power supply provides energy to CPU and I/O modules; CPU processes logic; Input modules receive signals from sensors; Output modules send signals to actuators.

Execution Sample

PLC Programming

Power Supply ON
CPU starts
Inputs read
CPU processes logic
Outputs activated

This sequence shows how PLC hardware components work step-by-step from power on to output activation.

Execution Table

Step	Component	Action	State Change	Output
1	Power Supply	Turns ON	Voltage supplied to CPU and I/O	Power available
2	CPU	Starts running	CPU initializes and runs program	Ready to process
3	Input Modules	Read signals	Input signals from sensors captured	Input data available
4	CPU	Processes logic	CPU executes control program using inputs	Decides outputs
5	Output Modules	Activate outputs	Outputs energized to actuators	Actuators respond
6	System	Waits for next cycle	CPU waits for next scan cycle	System stable

💡 PLC runs continuously in cycles until powered off

Variable Tracker

Component	Initial State	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	Final State
Power Supply	OFF	ON	ON	ON	ON	ON	ON
CPU	Idle	Idle	Running	Running	Processing	Processing	Waiting
Input Modules	No data	No data	No data	Data read	Data read	Data read	Data read
Output Modules	Off	Off	Off	Off	Off	Activated	Activated

Key Moments - 3 Insights

Why does the CPU need the power supply before it can start processing?

The power supply provides the necessary electrical energy for the CPU to operate, as shown in step 1 of the execution_table where power supply turns ON before CPU starts.

How do input modules affect the CPU's decision-making?

Input modules read signals from sensors and provide data to the CPU (step 3), enabling the CPU to process logic based on real-world conditions (step 4).

What happens if output modules do not activate after CPU processing?

If output modules fail to activate (step 5), actuators won't respond, meaning the control action is not executed despite CPU processing.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of the CPU after step 2?

A Idle

B Processing

C Running

D Waiting

💡 Hint

Check the 'State Change' column for CPU at step 2 in execution_table.

At which step do input modules provide data to the CPU?

A Step 3

B Step 1

C Step 4

D Step 5

💡 Hint

Look for when input signals are read in the execution_table.

If the power supply is OFF, what will be the state of the output modules?

A Activated

B Off

C Processing

D Running

💡 Hint

Refer to variable_tracker for power supply OFF state and output modules initial state.

Concept Snapshot

PLC hardware has three main parts:
- Power Supply: provides energy
- CPU: runs the control program
- I/O Modules: Input reads sensors, Output controls actuators
Power supply must be ON first,
CPU reads inputs, processes logic, then activates outputs.

Full Transcript

A PLC hardware system starts with the power supply turning ON, providing energy to the CPU and I/O modules. The CPU then starts running its control program. Input modules read signals from sensors and send this data to the CPU. The CPU processes this information and decides which outputs to activate. Output modules then energize actuators to perform actions. This cycle repeats continuously while the PLC is powered. Understanding this flow helps beginners see how each hardware part works together step-by-step.

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