Input addressing scheme in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Determine the input address value

Given a PLC input module with base address 0x100 and an input channel offset of 5, what is the hexadecimal address of the input channel?

A 0x105

B 0x150

C 0x110

D 0x115

💡 Hint

Add the offset to the base address in hexadecimal.

The input channel address is calculated by adding the offset (5) to the base address (0x100), resulting in 0x105.

🧠 Conceptual

intermediate

Identify the correct input addressing scheme

Which input addressing scheme correctly represents a digital input at byte 2, bit 3 in a PLC system?

A I2.3

B I3.2

C I2.5

D I1.3

💡 Hint

The format is I. where byte and bit are zero-based.

I2.3 means input byte 2, bit 3, which matches the question exactly.

🔧 Debug

advanced

Find the error in input address mapping

A PLC program uses the input address I1.8 to read a digital input. What is the issue with this address?

A The address should be I0.8 instead

B Byte number 1 is invalid; bytes start at 0

C Bit number 8 is invalid; bits range from 0 to 7 per byte

D No issue; I1.8 is valid

💡 Hint

Bits in a byte are numbered 0 through 7.

Bits in a byte are numbered 0 to 7, so bit 8 is invalid and causes an addressing error.

🚀 Application

advanced

Calculate input address for analog input channel

An analog input module starts at address 0x200 and each channel uses 2 bytes. What is the starting address of channel 4?

A 0x204

B 0x208

C 0x20A

D 0x20C

💡 Hint

Multiply the channel number by 2 bytes and add to base address.

Channel 4 means offset 4 * 2 = 8 bytes; 0x200 + 0x8 = 0x208.

🧠 Conceptual

expert

Understanding input addressing with byte and bit offsets

If a PLC input address is given as I3.6, which of the following statements is true?

A It refers to the 36th input channel

B It refers to byte 6, bit 3 of the input module

C It is an invalid address format

D It refers to byte 3, bit 6 of the input module

💡 Hint

The format I. means byte number then bit number.

I3.6 means byte 3, bit 6, which is a valid input address format.

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