Nexto XF – #41 – Creating a List of Persistent Variables – Knowledge Base

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

Components

Equipment: 1 computer/notebook.

Software: MasterTool X

Tutorial Sections

ARCHITECTURE
DEVELOPMENT

1. ARCHITECTURE

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

Figure 1. Computer running MasterTool X

2. DEVELOPMENT

Persistent variables are used when you want to ensure that certain values are permanently preserved, even in critical situations such as:

Complete PLC shutdown
Downloading new code
Project changes
Application reset

They are ideal for data that cannot be lost under any circumstances, such as calibration parameters, recipes, operator settings, or critical production data.

Difference between RETAIN and PERSISTENT variables

Variable type	Declaration block	Retains value after PLC shutdown	Retains value after code download	Can be changed at runtime	Where it can be created	Typical use
VAR (normal)	VAR	No	No	Yes	Local (UserPrg, POU) or Global (GVL)	Common logic, temporary signals
VAR RETAIN	VAR RETAIN VAR_GLOBAL RETAIN	Yes	No (can be lost)	Yes	Local or Global	States that need to survive power failure
VAR PERSISTENT	VAR_PERSISTENT	Yes	Yes	Yes	Only in PersistentVars	Critical parameters, calibration, recipes
VAR CONSTANT	VAR CONSTANT VAR_GLOBAL CONSTANT	Not applicable	Not applicable	No	Local or Global	Fixed values, limits, calculation

Important notes

Normal variable (VAR)
- Loses its value whenever the PLC restarts or the code is reloaded.
- It is the most used in everyday life.
RETAIN Variable
- Maintains value during power outages.
- Can be reset on full downloads or structural changes.
PERSISTENT Variable
- Maximum retention level.
- Always created in the PersistentVars object.
- Ideal for data that cannot be lost.
CONSTANT Variable
- Fixed value defined in the declaration.
- Cannot be written or forced.
- Does not occupy dynamic runtime memory.

Where persistent variables can be created

Unlike RETAIN variables, persistent variables can only be created in a specific location in the project.

They cannot be declared:

In UserPrg
In subroutines
In functional blocks
In functions
In common GVLs

How to create persistent variables (step by step)

In the project tree, locate the Application item
Right-click on Application
Select Add object
Choose PersistentVars

This object will be the only valid location for declaring persistent variables.

Figure 2. Creating the persistent variables menu

Declaration of persistent variables

Within the PersistentVars object, use the syntax:

VAR_GLOBAL PERSISTENT RETAIN
    
total_production: INT; 
    
active_revenue: INT; 
    
calibration_factor: REAL;
END_VAR

All variables declared in this block:

Are persistent
Are automatically retained
Maintain their values even after downloading new code

Figure 3. Creation of persistent variables

Use of persistent variables in the project

After being declared, persistent variables:

Can be used anywhere in the code
Function as global variables
Do not require any additional configuration
Access is direct by variable name.

Best practices in the use of persistent variables

Use only for truly critical data
Avoid storing temporary signals
Use clear and descriptive names
Document the reason for persistence
Avoid excessive persistent variables (non-volatile memory is limited)

Conclusion

Persistent variables (PERSISTENT) guarantee the highest level of data preservation in the PLC.

They should be used judiciously, only when the loss of information is unacceptable, and always created in the correct object (PersistentVars), respecting good industrial programming practices.

Figure 4. Use of persistent variables in UserPrg

Related to