Remanent variables can retain their values beyond the usual program runtime. Remanent variables can be declared as RETAIN variables or even more strictly as PERSISTENT variables in the PLC project.

A prerequisite for the full functionality of RETAIN variables is a corresponding memory area on the controller (NovRam). Persistent variables are written only when TwinCAT shuts down. This will require usually a corresponding UPS. Exception: Persistent variables can also be written with the FB_WritePersistentData function block.

If the corresponding memory area does not exist, the values of RETAIN and PERSISTENT variables are lost during a power failure.

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.

Persistent example:

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

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.

Retain example:

VAR RETAIN
    nRem1 : INT;
END_VAR