Input addressing scheme in PLC Programming - Step-by-Step Execution

Concept Flow - Input addressing scheme

Start

↓

Identify Input Device

↓

Assign Address

↓

Use Address in Program

↓

Read Input Value

↓

Process Input

↓

End

The input addressing scheme starts by identifying the input device, assigning it an address, then reading and processing the input value in the PLC program.

Execution Sample

PLC Programming

I0.0 := ReadInput(I0.0);
IF I0.0 = TRUE THEN
  Output := TRUE;
ELSE
  Output := FALSE;
END_IF;

This code reads the input at address I0.0 and turns an output ON if the input is TRUE, otherwise OFF.

Execution Table

Step	Action	Input Address	Input Value	Condition	Output Action
1	Read input	I0.0	TRUE	I0.0 = TRUE	Output := ON
2	Evaluate condition	I0.0	TRUE	Condition TRUE	Output ON executed
3	End of cycle	-	-	-	Program waits for next scan

💡 Input read and processed; program waits for next scan cycle.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
I0.0	FALSE	TRUE	TRUE	TRUE
Output	OFF	OFF	ON	ON

Key Moments - 2 Insights

Why do we use addresses like I0.0 for inputs?

Because each physical input device is mapped to a unique address like I0.0, so the PLC knows exactly which input to read. See execution_table step 1 where I0.0 is read.

What happens if the input value is FALSE?

The condition I0.0 = TRUE fails, so the output stays OFF. This is implied in execution_table step 2 where the condition is TRUE to turn output ON.

Visual Quiz - 3 Questions

Look at the execution table, what is the input value at step 1?

A TRUE

B FALSE

C UNKNOWN

D OFF

💡 Hint

Check the 'Input Value' column in execution_table row 1.

At which step does the output change to ON?

A Step 1

B Step 3

C Step 2

D Never

💡 Hint

Look at the 'Output Action' column in execution_table rows 1 and 2.

If the input at I0.0 was FALSE, what would happen to the output?

A Output turns ON

B Output stays OFF

C Program crashes

D Output toggles

💡 Hint

Refer to key_moments answer about input FALSE condition.

Concept Snapshot

Input addressing scheme:
- Inputs have unique addresses like I0.0
- PLC reads input value from address
- Use input address in program conditions
- Input TRUE triggers actions
- Input FALSE skips actions
- Addressing links physical input to code

Full Transcript

In PLC programming, each input device is assigned a unique address such as I0.0. The program reads the input value at this address during each scan cycle. If the input is TRUE, the program can trigger actions like turning an output ON. If the input is FALSE, the program skips those actions. This addressing scheme connects physical inputs to the program logic clearly and reliably.

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