10 Retain and Persistent variables
The PLC functionality have support for storing non-volatile retain and persistent variables
Those variabels can be stored dynamically in two different modes:
Power-fail mode:
If the voltage drops below a hardware given threshold (that is, during a power loss or
power dip), the system instantly copies all retain and persistent data defined in the PLC
application from RAM into the battery backed SRAM.
They are verified and restored when the device is restarted.
Cyclic mode:
the PLC is instructed to store all retain and persistent data directly into SRAM
synchronously with every PLC cycle. This means, there is no restriction about the
memory size and all device types can use the whole available memory, which is 128 kiB.
However, writing to the SRAM might increase the configured PLC cycle depending on
the size of used retain and persistent data, because a SRAM is naturally slower then a
RAM.
Overview of Retain/persistant data configurations:
Refer to documentation for details:
“How to use Retain/Persistent data in CoDeSys on X2 control/motion/marine SC”
Persistent variables (non-volatile memory) can ONLY be declared in a special global
variables list, which is assigned to an application. There might be only ONE such list per
application.
Highlight Application.
Right click and select Add Object.
Choose PersistentVariables..
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