Nexto XF – #40 Creating a List of Retentive Variables

The objective of this tutorial is to explain how to create a list of retentive variables in a MasterTool X project.

Components

Equipment: 1 computer/notebook.

Software: MasterTool X

Tutorial Sections

1. ARCHITECTURE
2. DEVELOPMENT

1. ARCHITECTURE

A computer/notebook will be required to use MasterTool X.

Figure 1. Computer running MasterTool X

2. DEVELOPMENT 

Retentive variables are used when you want a value not to be lost in situations such as:

• PLC shutdown
• Power outage
• Controlled CPU reset

That is, the variable value is kept in non-volatile memory and restored when the controller resumes operation.

How Retain Variables Work

• The value is automatically saved by the PLC
• Upon restart, the previous value is restored
• No additional code is needed to maintain the value
• Retention depends on the controller's resources

Excessive use of retainable variables can impact available memory.

When to use retainable variables

Use RETAIN variables for:

• Production counters
• Operating states
• Operator-adjusted parameters
• Essential process history

Avoid using RETAIN for:

• Temporary variables
• Instantaneous signals
• Logic that doesn't need history

Where can retain variables be created?

Retainable variables can be declared in any project scope, including:

• Global variables (GVL)
• Local variables (UserPrg, subroutines, functional blocks, or functions)

The most important point is not where the variable is located, but the correct declaration syntax.

Syntax for Retentive Global Variables

To create retentive global variables, use the block:

VAR_GLOBAL RETAIN
    total_counter : DINT;
    system_state : BOOL;
END_VAR

All variables declared within this block will retain their values.

Figure 2. Creation of global and retentive variables

Syntax for retentive local variables

In UserPrg, subroutines, functional blocks, or functions:

VAR RETAIN
    operation_time : TIME;
    execution_cycles : DINT;
END_VAR

These variables will retain their values ​​even after shutdown or reset.

Figure 3. Creation of local and retentive variables

Best practices

• Use RETAIN only when necessary
• Clearly name retentive variables
• Document usage with comments
• Prefer GVL for shared retentive data

Conclusion

Retentive variables (RETAIN) are fundamental to ensuring process continuity and reliability, allowing important information to be preserved even after terminations, provided they are used judiciously and with good organization.