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

Recall & Review

beginner

What is an input addressing scheme in PLC programming?

It is a method to identify and access physical input devices connected to a PLC using specific addresses.

beginner

Why do PLCs use input addressing schemes?

To organize and read signals from many input devices clearly and efficiently, like sensors or switches.

beginner

How is an input address typically formatted in a PLC?

Usually as a letter and number combination, for example, I0.0 means input module 0, bit 0.

intermediate

What does the address I1.3 represent in a PLC input addressing scheme?

It represents the input at module 1, bit 3, meaning the fourth input on the second input module.

intermediate

How can understanding input addressing help in troubleshooting PLC systems?

It helps locate which physical input device corresponds to a signal in the program, making fixes faster.

What does the 'I' in an input address like I0.2 stand for?

A Instruction

B Internal

C Input

D Indicator

The 'I' stands for Input, indicating the address is for an input device.

In the address I2.5, what does the number '5' represent?

A The input module number

B The bit or channel number on the module

C The output number

D The program step

The '5' is the bit or channel number on input module 2.

Why is it important to use a consistent input addressing scheme?

A To increase power consumption

B To confuse users

C To slow down the PLC

D To make programming and troubleshooting easier

Consistency helps programmers and technicians understand and fix the system quickly.

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

A I0.7

B Q0.1

C M2.3

D O1.0

'I0.7' is a valid input address; 'O' and 'Q' usually refer to outputs, 'M' to memory.

If a sensor is connected to input I0.1, what does this mean?

A The sensor is connected to input module 0, bit 1

B The sensor is connected to output 1

C The sensor is connected to memory bit 1

D The sensor is connected to input module 1, bit 0

I0.1 means input module 0, bit 1, where the sensor signal is read.

Explain what an input addressing scheme is and why it is important in PLC programming.

Think about how PLC knows which sensor or switch is which.

Describe how to interpret the input address I3.4 in a PLC system.

Break down the address into parts and explain each.

Practice

1. What does the input addressing scheme I1.3 mean in a PLC program?

easy

A. Internal memory at byte 1, bit 3

B. Output at byte 1, bit 3

C. Input at byte 3, bit 1

D. Input at byte 1, bit 3

Solution

1. Step 1: Understand the input address format

   The format I<byte>.<bit> means input at specified byte and bit.

2. Step 2: Interpret I1.3

   This means input at byte 1, bit 3.

3. Final Answer:

   Input at byte 1, bit 3 -> Option D

4. Quick Check:

   Input address = I1.3 means byte 1, bit 3 [OK]

Hint: I means input, number before dot is byte, after dot is bit [OK]

Common Mistakes:

• Confusing input with output addresses
• Mixing up byte and bit positions
• Assuming bit number comes before byte

2. Which of the following is the correct syntax for addressing input bit 5 in byte 2?

easy

A. I2.5

B. I5.2

C. O2.5

D. I2-5

Solution

1. Step 1: Recall input address syntax

   The correct format is I<byte>.<bit>.

2. Step 2: Match byte 2 and bit 5

   So input bit 5 in byte 2 is I2.5.

3. Final Answer:

   I2.5 -> Option A

4. Quick Check:

   Correct input syntax = I2.5 [OK]

Hint: Use dot between byte and bit, start with I for input [OK]

Common Mistakes:

• Swapping byte and bit numbers
• Using O instead of I for input
• Using dash instead of dot

3. Given the PLC input address I0.7 is ON and all others OFF, what is the binary value of byte 0?

medium

A. 00000001

B. 10000000

C. 11111111

D. 00000000

Solution

1. Step 1: Understand bit numbering in a byte

   Bit 0 is the least significant bit (rightmost), bit 7 is the most significant (leftmost).

2. Step 2: Set bit 7 ON and others OFF

   Bit 7 ON means the leftmost bit is 1, others 0, so binary is 10000000.

3. Final Answer:

   10000000 -> Option B

4. Quick Check:

   I0.7 ON means bit 7 = 1, binary = 10000000 [OK]

Hint: Bit 7 is leftmost bit in byte, so 10000000 [OK]

Common Mistakes:

• Assuming bit 7 is rightmost bit
• Confusing bit numbering direction
• Setting wrong bit ON

4. A PLC program uses the input address I3.8. What is wrong with this address?

medium

A. Bit number 8 does not exist in a byte

B. Byte number 3 is invalid

C. Input addresses cannot have dots

D. I should be lowercase

Solution

1. Step 1: Recall bit numbering limits

   A byte has bits numbered 0 to 7 only, so bit 8 is invalid.

2. Step 2: Check address I3.8

   Bit 8 is outside valid range, so address is incorrect.

3. Final Answer:

   Bit number 8 does not exist in a byte -> Option A

4. Quick Check:

   Bits 0-7 valid, 8 invalid [OK]

Hint: Bits go 0 to 7 only, no bit 8 in a byte [OK]

Common Mistakes:

• Thinking bits go 1 to 8
• Ignoring dot separator
• Assuming case sensitivity matters

5. You want to read inputs from byte 4 bits 0 to 3 and combine them into a 4-bit number in your PLC program. Which input addresses should you use?

hard

A. I4.4, I4.5, I4.6, I4.7

B. I0.4, I1.4, I2.4, I3.4

C. I4.0, I4.1, I4.2, I4.3

D. I3.0, I3.1, I3.2, I3.3

Solution

1. Step 1: Identify byte and bit range

   You want bits 0 to 3 in byte 4, so bits 0,1,2,3 of byte 4.

2. Step 2: Write input addresses for these bits

   These are I4.0, I4.1, I4.2, I4.3.

3. Final Answer:

   I4.0, I4.1, I4.2, I4.3 -> Option C

4. Quick Check:

   Bits 0-3 in byte 4 = I4.0 to I4.3 [OK]

Hint: Bits 0-3 in byte 4 are I4.0 to I4.3 [OK]

Common Mistakes:

• Mixing bits from different bytes
• Using bits 4-7 instead of 0-3
• Confusing byte numbers