Remanent Variables - PERSISTENT, RETAIN

Persistent Variables

You can declare persistent variables by adding the keyword PERSISTENT after the keyword for the variable type (VAR, VAR_GLOBAL, etc.) in the declaration part of programming objects.

PERSISTENT variables retain their value after an uncontrolled termination, a Reset cold or a new download of the PLC project.
When the program restarts, the system continues to operate with the stored values. In this case TwinCAT reinitializes "normal" variables with their explicitly specified initial values or with the default initializations.
In other words, TwinCAT only reinitializes PERSISTENT variables during a Reset origin.

An application example for persistent variables is an operating hour counter, which is to continue counting after a power failure and when the PLC project is downloaded again.

Overview table showing the behavior of PERSISTENT variables

Sample:

Persistent variable list:

VAR_GLOBAL PERSISTENT
    nVarPers1 : DINT; (* 1. Persistent variable *)
    bVarPers2 : BOOL; (* 2. Persistent variable *)
END_VAR

Avoid using the POINTER TO data type in persistent variable lists, since the address values ​​may change when the PLC project is downloaded again! TwinCAT issues corresponding compiler warnings. Declaring a local variable as a PERSISTENT in a function has no effect. Data persistence cannot be used in this way. The behavior during a cold reset can be influenced with the pragma 'TcInitOnReset'

RETAIN variables

You can declare RETAIN variables by adding the keyword RETAIN after the keyword for the variable type (VAR, VAR_GLOBAL, etc.) in the declaration part of programming objects.

Variables declared as RETAIN are target system-dependent, but typically managed in a separate memory area that must be protected against power failure. The so-called Retain handler ensures that the RETAIN variables are written at the end of a PLC cycle and only in the corresponding area of the NovRam. The handling of the retain handler is described in chapter "Retain data" of the C/C++ documentation.

RETAIN variables retain their value after an uncontrolled termination (power failure). When the program restarts, the system continues to operate with the stored values. In this case TwinCAT reinitializes "normal" variables with their explicitly specified initial values or with the default initializations.
TwinCAT reinitializes RETAIN variables at a reset origin.

One possible application is a part counter in a production plant, which should continue to count after a power failure.

Overview table showing the behavior of RETAIN variables

Samples:

In a POU:

VAR RETAIN
    nRem1 : INT;
END_VAR

In a GVL:

VAR_GLOBAL RETAIN
    nVarRem1 : INT;
END_VAR

If you declare a local variable as a RETAIN in a program or a function block, TwinCAT stores this specific variable in the retain area (like a global RETAIN variable). If you declare a local variable in a function as RETAIN, this has no effect. TwinCAT does not store the variable in the retain area.

Complete overview table

The degree of retention of RETAIN variables is automatically included in that of PERSISTENT variables.