Output addressing scheme in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an output addressing scheme in PLC programming?

It is a method to identify and access output devices connected to the PLC using specific addresses.

beginner

Why do PLCs use output addressing schemes?

To control devices like motors or lights by sending signals to the correct output point.

beginner

In a PLC, what does an output address like Q0.0 represent?

It represents the first output point (bit 0) on the first output byte (Q0) of the PLC.

beginner

How does the output addressing scheme help in real-life automation?

It lets the PLC know exactly which device to turn on or off, like a conveyor belt motor or a warning light.

intermediate

What is the difference between byte and bit in output addressing?

A byte groups 8 bits; each bit controls one output point. The address shows both byte and bit to pinpoint the output.

What does the 'Q' in an output address like Q0.3 stand for?

A Output

B Input

C Memory

D Timer

In PLC addressing, 'Q' denotes an output point.

If a PLC output address is Q1.5, which output bit is being addressed?

A Bit 1 of byte 0

B Bit 1 of byte 5

C Bit 5 of byte 0

D Bit 5 of byte 1

Q1.5 means byte 1, bit 5.

Why is output addressing important in PLC programming?

A To read sensor data

B To control specific devices connected to outputs

C To store program code

D To debug the PLC

Output addressing lets the PLC control the right device.

Which of these is a valid output address in a typical PLC?

A I0.0

B M1.0

C Q0.7

D T2

'Q0.7' is a valid output address; 'I' is input, 'M' is memory, 'T' is timer.

How many output points can one byte address in a PLC output module control?

A 8

B 4

C 16

D 1

One byte has 8 bits, so it can control 8 output points.

Explain how output addressing works in a PLC and why it is important for automation.

Think about how the PLC tells a motor or light to turn on.

Describe the difference between input and output addressing in PLCs.

Inputs read sensors; outputs control devices.

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