PLC program documentation standards in PLC Programming

Introduction

Good documentation helps everyone understand and use PLC programs easily. It makes fixing and improving programs faster and safer.

When creating a new PLC program for a machine or process.

When updating or changing an existing PLC program.

When handing over a PLC program to another technician or team.

When troubleshooting or fixing problems in a PLC-controlled system.

When training new staff on how the PLC program works.

Syntax

PLC Programming

Documentation in PLC programs usually includes comments, labels, and structured notes.

Example:
// This section controls the motor start
Start_Motor := Input_Start AND NOT Motor_Running;

(*
  Description: Controls the conveyor belt speed
  Author: John Doe
  Date: 2024-06-01
*)

Use clear and simple language in comments.

Keep comments close to the code they explain.

Examples

A single line comment explaining the logic next to the code.

PLC Programming

// Start motor when start button is pressed
Start_Motor := Input_Start AND NOT Motor_Running;

A multi-line comment block giving more detailed information.

PLC Programming

(*
  This block controls the conveyor speed.
  Adjust speed with Speed_Setpoint variable.
*)
Conveyor_Speed := Speed_Setpoint;

Using labels to organize code sections and comments to explain.

PLC Programming

Label: Motor_Control
// Controls motor start and stop
Start_Motor := Input_Start AND NOT Motor_Running;
Stop_Motor := Input_Stop OR Motor_Fault;

Sample Program

This simple PLC program controls a motor start and stop with clear comments explaining each part.

PLC Programming

// PLC program example with documentation
// Author: Jane Smith
// Date: 2024-06-01

// Input: Start button
// Output: Motor control

// Start motor when start button is pressed and motor is not running
Start_Motor := Input_Start AND NOT Motor_Running;

// Stop motor when stop button is pressed or fault occurs
Stop_Motor := Input_Stop OR Motor_Fault;

// Motor running status
Motor_Running := Start_Motor AND NOT Stop_Motor;

Important Notes

Always update documentation when you change the program.

Use consistent naming for variables and labels.

Keep comments short but clear to avoid clutter.

Summary

Documentation makes PLC programs easier to understand and maintain.

Use comments and labels to explain what the code does.

Keep documentation clear, simple, and up to date.

Practice

1. Why is it important to include documentation in a PLC program?

easy

A. It automatically fixes errors in the program.

B. It makes the program run faster.

C. It reduces the size of the program file.

D. It helps others understand and maintain the program easily.

Solution

1. Step 1: Understand the purpose of documentation

   Documentation explains the program's purpose and logic to others.

2. Step 2: Identify benefits of documentation

   It helps maintain and update the program without confusion.

3. Final Answer:

   It helps others understand and maintain the program easily. -> Option D

4. Quick Check:

   Documentation improves understanding = A [OK]

Hint: Documentation = easier understanding and maintenance [OK]

Common Mistakes:

• Thinking documentation speeds up program execution
• Assuming documentation reduces program size
• Believing documentation fixes errors automatically

2. Which of the following is the correct way to add a comment in a PLC program?

easy

A. (* This is a comment *)

B. # This is a comment

C. // This is a comment

D.

Solution

1. Step 1: Identify comment syntax in PLC programming

   Many PLC languages use (* ... *) for comments.

2. Step 2: Compare options

   // is for C-like languages, # for scripting, for HTML, so only (* ... *) fits PLC.

3. Final Answer:

   (* This is a comment *) -> Option A

4. Quick Check:

   PLC comments use (* ... *) = C [OK]

Hint: PLC comments use (* and *) symbols [OK]

Common Mistakes:

• Using // or # which are not standard in PLC
• Confusing HTML comments with PLC comments
• Leaving out comment delimiters

3. Given this comment in a PLC program: (* Input: Start button, Output: Motor *), what is the main purpose of this comment?

medium

A. To optimize the motor control logic.

B. To execute the motor when the start button is pressed.

C. To describe the program's inputs and outputs clearly.

D. To declare variables for the start button and motor.

Solution

1. Step 1: Read the comment content

   The comment states inputs and outputs: Start button and Motor.

2. Step 2: Understand comment purpose

   Comments explain code parts, not execute or declare variables.

3. Final Answer:

   To describe the program's inputs and outputs clearly. -> Option C

4. Quick Check:

   Comments explain inputs/outputs = D [OK]

Hint: Comments explain, not execute or declare [OK]

Common Mistakes:

• Thinking comments run code
• Confusing comments with variable declarations
• Assuming comments optimize logic

4. A PLC program has this comment: (* Author: John Doe Purpose: Control conveyor speed *) but it is missing input and output details. What is the best fix?

medium

A. Remove the author name to save space.

B. Add comments describing inputs and outputs clearly.

C. Delete the comment because it is incomplete.

D. Add code to automatically generate comments.

Solution

1. Step 1: Identify missing documentation parts

   The comment lacks input and output descriptions which are important.

2. Step 2: Choose the best way to improve documentation

   Adding clear input/output comments improves understanding and maintenance.

3. Final Answer:

   Add comments describing inputs and outputs clearly. -> Option B

4. Quick Check:

   Complete comments include inputs/outputs = B [OK]

Hint: Complete comments include inputs and outputs [OK]

Common Mistakes:

• Removing author info unnecessarily
• Deleting useful comments
• Expecting code to auto-generate comments

5. You need to document a complex PLC program controlling multiple machines. Which approach best follows PLC program documentation standards?

hard

A. Write clear comments for each machine's inputs, outputs, logic, and author info.

B. Write one long comment at the start describing the whole program.

C. Skip comments to avoid clutter and rely on variable names only.

D. Use technical jargon to impress other engineers.

Solution

1. Step 1: Understand documentation needs for complex programs

   Complex programs need clear, organized comments for each part.

2. Step 2: Evaluate options for clarity and standards

   Clear comments per machine with inputs, outputs, logic, and author info follow best practices.

3. Final Answer:

   Write clear comments for each machine's inputs, outputs, logic, and author info. -> Option A

4. Quick Check:

   Clear, organized comments = A [OK]

Hint: Document each part clearly with inputs, outputs, logic [OK]

Common Mistakes:

• Writing only one big comment
• Skipping comments entirely
• Using confusing technical jargon