Output addressing scheme in PLC Programming

Introduction

An output addressing scheme tells the PLC where to send signals to control devices like lights or motors.

When you want to turn on a motor using a PLC.

When you need to control a light or indicator from the PLC.

When you want to activate a valve or relay through the PLC.

When wiring outputs to physical devices in an automation system.

When programming the PLC to control external equipment.

Syntax

PLC Programming

Output_Address = Output_Module_Number.Output_Point_Number

Output addresses usually start with a letter like Q or O depending on the PLC brand.

The address points to a specific output terminal on the PLC hardware.

Examples

This means output module 0, point 0. It controls the first output device.

PLC Programming

Q0.0

This means output module 1, point 3. It controls the fourth output device on module 1.

PLC Programming

Q1.3

Some PLCs use 'O' instead of 'Q' for outputs. This is output 5 on module 0.

PLC Programming

O0.5

Sample Program

This program checks if input I0.0 is ON. If yes, it turns ON output Q0.0. Otherwise, it turns it OFF.

PLC Programming

(* Simple PLC output control example *)
(* Turn on output Q0.0 when input I0.0 is ON *)
IF I0.0 THEN
  Q0.0 := TRUE;
ELSE
  Q0.0 := FALSE;
END_IF;

Important Notes

Output addresses must match the physical wiring on the PLC hardware.

Always check your PLC manual for the correct output address format.

Using the wrong output address can cause devices not to work or safety issues.

Summary

Output addressing scheme tells the PLC where to send control signals.

It uses a format like Q0.0 to point to specific output points.

Correct addressing is important for safe and proper device control.

Practice

1. What does the output addressing scheme in a PLC primarily specify?

easy

A. Which physical output device the PLC controls

B. The input device connected to the PLC

C. The programming language used in the PLC

D. The power supply voltage for the PLC

Solution

1. Step 1: Understand output addressing purpose

   Output addressing tells the PLC which output device to activate or control.

2. Step 2: Differentiate from inputs and other settings

   It does not specify inputs, programming language, or power supply.

3. Final Answer:

   Which physical output device the PLC controls -> Option A

4. Quick Check:

   Output addressing = physical output device [OK]

Hint: Output addressing points to the device PLC controls [OK]

Common Mistakes:

• Confusing output with input addressing
• Thinking it defines programming language
• Mixing it with power supply details

2. Which of the following is a correct format for output addressing in a PLC?

easy

A. I:3/7

B. O:2/5

C. M:1/0

D. T:4/2

Solution

1. Step 1: Identify output address format

   Output addresses usually start with 'O' followed by module and point numbers, e.g., O:2/5.

2. Step 2: Recognize other prefixes

   'I' is for inputs, 'M' for memory bits, 'T' for timers, so they are incorrect for output addressing.

3. Final Answer:

   O:2/5 -> Option B

4. Quick Check:

   Output address starts with 'O' [OK]

Hint: Output addresses start with 'O:' followed by module/point [OK]

Common Mistakes:

• Choosing input or memory addresses as output
• Mixing timer addresses with outputs
• Ignoring the prefix letter

3. Given the output address O:1/3, which physical output does it refer to?

medium

A. Module 1, output point 3

B. Module 3, output point 1

C. Input module 1, point 3

D. Memory bit 1, point 3

Solution

1. Step 1: Parse the output address format

   O:1/3 means output module 1, point 3 (the third output on module 1).

2. Step 2: Exclude other interpretations

   It is not a memory bit, not an input module, and the format is module/point (not point/module).

3. Final Answer:

   Module 1, output point 3 -> Option A

4. Quick Check:

   O:1/3 = module 1, point 3 [OK]

Hint: Format is O:module/point, read left to right [OK]

Common Mistakes:

• Swapping module and point numbers
• Confusing output with input addresses
• Ignoring the prefix 'O:'

4. Identify the error in this output address: O:2-4

medium

A. Output point cannot be 4

B. Module number cannot be 2

C. Incorrect separator; should use '/' instead of '-'

D. Prefix 'O' is invalid

Solution

1. Step 1: Check the separator in the address

   Output addresses use a slash '/' to separate module and point, not a dash '-'.

2. Step 2: Validate other parts

   Module 2 and point 4 are valid numbers, and 'O' prefix is correct for output.

3. Final Answer:

   Incorrect separator; should use '/' instead of '-' -> Option C

4. Quick Check:

   Use '/' between module and point [OK]

Hint: Use '/' not '-' between module and point [OK]

Common Mistakes:

• Using dash instead of slash
• Thinking module or point numbers are invalid
• Misunderstanding the prefix letter

5. You have a PLC with 3 output modules. Module 0 has 8 points, module 1 has 8 points, and module 2 has 4 points. Which output address correctly refers to the last output point of module 2?

hard

A. O:1/7

B. O:2/4

C. O:3/0

D. O:2/3

Solution

1. Step 1: Understand module points numbering

   Points start at 0, so module 2 with 4 points has points 0,1,2,3.

2. Step 2: Identify last point of module 2

   The last point is 3, so address is O:2/3. O:2/4 is invalid (point 4 does not exist).

3. Step 3: Exclude other options

   O:3/0 is invalid (module 3 does not exist). O:1/7 is last point of module 1, not module 2.

4. Final Answer:

   O:2/3 -> Option D

5. Quick Check:

   Last point = points - 1 = 3 [OK]

Hint: Last point = total points minus one [OK]

Common Mistakes:

• Counting points starting at 1 instead of 0
• Using invalid module numbers
• Confusing module and point numbers