Register and data table concepts in PLC Programming - Step-by-Step Execution

Concept Flow - Register and data table concepts

Start

↓

Define Registers

↓

Assign Values to Registers

↓

Create Data Table

↓

Store Registers in Data Table

↓

Access Data Table Values

↓

Use Values in Logic

↓

End

This flow shows how registers are defined and assigned values, then stored in a data table, which is accessed later for logic operations.

Execution Sample

PLC Programming

REG1 = 10
REG2 = 20
DATA_TABLE = [REG1, REG2]
OUTPUT = DATA_TABLE[1]

Assign values to two registers, store them in a data table, then access the second value.

Execution Table

Step	Action	Register/Variable	Value	Output
1	Assign 10 to REG1	REG1	10
2	Assign 20 to REG2	REG2	20
3	Create DATA_TABLE with REG1 and REG2	DATA_TABLE	[10, 20]
4	Access DATA_TABLE at index 1	OUTPUT	20	20
5	End of execution

💡 All steps completed, OUTPUT holds value 20 from DATA_TABLE index 1

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
REG1	undefined	10	10	10	10	10
REG2	undefined	undefined	20	20	20	20
DATA_TABLE	undefined	undefined	undefined	[10, 20]	[10, 20]	[10, 20]
OUTPUT	undefined	undefined	undefined	undefined	20	20

Key Moments - 3 Insights

Why does OUTPUT get the value 20 and not 10?

Because DATA_TABLE[1] accesses the second element in the list (indexing starts at 0), which is 20 as shown in execution_table step 4.

Can we change a register value after storing it in the data table?

Yes, but the data table will keep the old value unless updated again. The execution_table shows values assigned before creating DATA_TABLE.

What happens if we try to access an index outside DATA_TABLE?

It would cause an error or return undefined because the data table only has two elements at indexes 0 and 1.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the value of DATA_TABLE?

A [20, 10]

B [10, 20]

C [10]

D undefined

💡 Hint

Check the 'Value' column for DATA_TABLE at step 3 in execution_table.

At which step does OUTPUT get assigned a value?

A Step 2

B Step 1

C Step 4

D Step 3

💡 Hint

Look for OUTPUT in the 'Register/Variable' column in execution_table.

If REG1 was changed to 30 after step 3, what would DATA_TABLE contain?

A [10, 20]

B [30]

C [30, 20]

D undefined

💡 Hint

DATA_TABLE was created at step 3 with values [10, 20], changing REG1 later does not update DATA_TABLE automatically.

Concept Snapshot

Registers hold single values.
Data tables store multiple values like lists.
Assign registers first, then create data tables.
Access data table by index starting at 0.
Changing registers after data table creation does not update table automatically.

Full Transcript

This lesson shows how registers and data tables work in PLC programming. First, registers are assigned values. Then, these values are stored in a data table, which is like a list holding multiple values. You can access values in the data table by their position, starting at zero. The example assigns 10 to REG1 and 20 to REG2, then creates a data table with these values. Accessing the second value in the data table gives 20. Changing a register after creating the data table does not change the table's stored values unless updated again. This helps organize and use multiple values efficiently in automation logic.

Practice

1. What is the primary purpose of a register in PLC programming?

easy

A. To store a single piece of data or value

B. To organize multiple registers into a table

C. To execute a sequence of instructions

D. To display output on a screen

Solution

1. Step 1: Understand what a register holds

   A register stores one piece of data, like a number or a status.

2. Step 2: Differentiate registers from data tables

   Data tables group many registers, but a single register holds only one value.

3. Final Answer:

   To store a single piece of data or value -> Option A

4. Quick Check:

   Register = single data storage [OK]

Hint: Registers hold one value; tables hold many [OK]

Common Mistakes:

• Confusing registers with data tables
• Thinking registers execute instructions
• Assuming registers display output

2. Which of the following is the correct syntax to declare a data table named DataTable1 with 10 registers in a typical PLC program?

easy

A. DataTable1: ARRAY[1..10] OF INT;

B. DataTable1 = {10 registers};

C. DECLARE DataTable1 WITH 10 REGISTERS;

D. DataTable1: INT[10];

Solution

1. Step 1: Identify correct array declaration syntax

   In PLC, arrays are declared with ARRAY and index range, e.g., ARRAY[1..10] OF INT.

2. Step 2: Check each option's syntax

   DataTable1: ARRAY[1..10] OF INT; matches standard PLC array declaration; others are incorrect or invalid syntax.

3. Final Answer:

   DataTable1: ARRAY[1..10] OF INT; -> Option A

4. Quick Check:

   ARRAY[1..10] OF INT is correct syntax [OK]

Hint: Use ARRAY with index range and type for tables [OK]

Common Mistakes:

• Using curly braces instead of ARRAY
• Missing index range in declaration
• Incorrect keyword usage like DECLARE

3. Given the following PLC code snippet, what is the value stored in DataTable1[3] after execution?

DataTable1: ARRAY[1..5] OF INT := [10, 20, 30, 40, 50];
DataTable1[3] := DataTable1[2] + DataTable1[4];

medium

A. 30

B. 60

C. 50

D. 70

Solution

1. Step 1: Identify initial values in DataTable1

   DataTable1 = [10, 20, 30, 40, 50], so DataTable1[2] = 20 and DataTable1[4] = 40.

2. Step 2: Calculate new value for DataTable1[3]

   DataTable1[3] = 20 + 40 = 60 after assignment.

3. Final Answer:

   60 -> Option B

4. Quick Check:

   20 + 40 = 60 [OK]

Hint: Add values at indexes 2 and 4 for index 3 [OK]

Common Mistakes:

• Using old value of DataTable1[3]
• Adding wrong indexes
• Confusing array indexing starting point

4. Identify the error in this PLC code snippet that tries to assign a value to a register:

Register1: INT;
Register1 := 100
Register2 := 200;

medium

A. Incorrect assignment operator used

B. Missing semicolon after first assignment

C. Register2 is not declared before use

D. Register1 should be declared as ARRAY

Solution

1. Step 1: Check declarations of registers

   Register1 is declared, but Register2 is used without declaration.

2. Step 2: Verify syntax and declarations

   Missing semicolon after first assignment is an error but not the main one; Register2 usage without declaration causes error.

3. Final Answer:

   Register2 is not declared before use -> Option C

4. Quick Check:

   Undeclared variable usage = error [OK]

Hint: Always declare registers before assigning values [OK]

Common Mistakes:

• Ignoring undeclared variables
• Assuming semicolon is optional
• Confusing assignment operator

5. You want to track the temperature readings from 8 sensors using registers and a data table. Which approach is best to organize this data efficiently?

hard

A. Create a data table with 1 register and update it 8 times

B. Create 8 separate registers named Temp1 to Temp8

C. Use a single register to store all 8 temperatures as a string

D. Use a data table (array) of 8 registers named TempSensors

Solution

1. Step 1: Consider data organization for multiple sensors

   Using a data table (array) groups related data efficiently and simplifies access.

2. Step 2: Evaluate options for best practice

   Separate registers are harder to manage; single register as string complicates processing; updating one register repeatedly loses data.

3. Final Answer:

   Use a data table (array) of 8 registers named TempSensors -> Option D

4. Quick Check:

   Data table groups multiple registers efficiently [OK]

Hint: Group similar data in arrays for easy management [OK]

Common Mistakes:

• Using many separate registers instead of array
• Storing multiple values in one register as string
• Overwriting single register repeatedly