For the generic Linux SL products, there are different possibilities to support retain/persistent variables or persistent storage of information/variables.

"Classic" Retains:

1) Store Retains in File
2) Store Retains in special memory (e.g. SRAM / NVRAM...)
3) Store Retains in shared memory

Besides this, it is also possible to use other features of CODESYS to better fulfill your needs, as they provide a higher level of persistence that we highly recommend:

1) Store Retains in File

The runtime requires either a proper shutdown to stop the application and then stores the retain variables in a file, or you can also trigger the storage manually inside the IEC application via the IEC library "CmpApp" → "AppStoreRetainsInFile"


Pro:

  • Easy to configure (is default)
  • No special memory/HW required
  • Single data consistency: ok (if finished properly/graceful shutdown) or perfect if manually triggered inside your IEC application

Con:

  • Processing time might be long
  • No "logical" consistency possible, except you trigger the "storage" by yourself: see "CmpApp" → AppStoreRetainsInFile() library function!


Configuration:

Nothing to do, as this is the default!


2) Store Retains in special memory

You can configure the runtime via the cfg File (CODESYSControl.cfg) so that it directly uses a given memory (e.g. physically mapped).

Pro:

  • Easy to configure
  • Processing time fast
    IEC Code directly accesses the retain variables inside the special memory
  • Single data consistency (for standard datatypes smaller or equal the architecture size (e.g. 32 bits)) ok
  • no further "glue code" required for storage

Con:

  • Single data consistency (for data bigger than architecture size (e.g. 32 bits)): no guarantee possible
  • Special memory/HW required (Kerneldriver etc.)
  • No "logical" consistency possible


Configuration:

[CmpApp]
RetainType.Application=InSRAM

[CmpRetain]
Retain.SRAM.Size=0x....
Retain.SRAM.AddressMapped= -> in case you have a mapped address or:
Retain.SRAM.Address= -> in case you have a physical address


3) Store Retains in shared memory

You can configure the runtime via the cfg File (CODESYSControl.cfg) so that it uses a shared memory memory. Then you can easily adapt to whatever "real" retain storage you have.

See also the following help page:

https://help.codesys.com/webapp/_rtslext_retains_in_shared_memory;product=CODESYS_Control_SL_Extension_Package;version=4.4.0.0


Pro:

  • Easy to configure
  • Processing time fast
  • No special memory/HW required, but is possible

Con:

  • Single data consistency: no guarantee possible
  • A bit of "glue code" is required to put the retains from shared memory to whatever real retain storage you have.


Configuration:

[CmpApp]
RetainType.Application=InSHM

[CmpRetain]
Retain.SHM.Size=0x....
Retain.SHM.Name= -> Filename of the shared memory to use


See also....