Question:
How can you create PERSISTENT variables?
Answer:
The Servo PLC (as of V6.3) and the ECS axis module (as of V6.5) are equipped with a PERSISTENT memory (see article
Implementation of the VAR Persistent memory class; Doc-ID 20034021). This is a fail-safe memory which, unlike the RETAIN memory, is not initialised even when downloading a project. The PERSISTENT memory can only be deleted in online mode by using the menu command
Online>Reset (original).
RETAIN variables must be declared between the keywords
VAR RETAIN and
END_VAR. Example:
VAR RETAIN
dnVarRetain : DINT;
END_VAR
Access to the persistent memory is effected via the system block
VAR_Persistent. For this, the system block must be added to the
PLC configuration (
Ressources tab) (select menu command
Add>Add subelement,
VAR_Persistent).
As persistent variable you can then either use one of the system variables contained in the system block
VAR_Persistent or you can declare a variable with the absolute address of the system block. Example:
VAR
dnVarPersistent AT %QD171.0 : DINT;
END_VAR
Note:
When declaring variables with an absolute address, it must be avoided that several variables accidentally use the same absolute memory area.
