Siemens Memory Structure

Quick question for you guys familiar with the memory structure of Siemens PLCs:

When I download the program to the PLC, it goes into the load memory. This includes all data blocks, FB's, FC's, etc.

During operation, the PLC copies relevant parts of the program from the load memory into the work memory, essentially RAM.

Is anything ever actually written back to the load memory during operation? Or is everything just processed in the work memory? For example, are data block bytes necessary for the program operation simply stored and processed entirely in the work memory? I thought I had read that the values of retentative data block bits get written back to the load memory upon loss of power, but does data get written there other times too?

Interestingly, the Easybook for the S7-1200 says that retentative memory is "non-volatile storage for a limited quantity of work memory values". Memory bits stored in system memory can be retentitive as well, but I thought that all memory bits were stored in the system memory.

Thanks for any clarification!

Hi Jieve.

Yes, Siemens do write in the documentation that the actual values of retentive DBs gets written to the MMC card upon power-down.
But, I know for sure that if you move the flash-card to another CPU, the CPU detects that there is a difference upon power-up, and promptly resets the actual values to the initial values.
Not sure what the whole idea is with the MMC card if you are not supposed to be able to move the MMC card from a defective CPU to a new CPU and keep the last good actual values.

The times that the MMC card is written:
When downloading blocks.
When performing a RAM-to-ROM.
When the SFC block WRIT_DBL is used.
When the CPU is powered down (alledgedly !).

I normally try to keep the good actual values by doing a RAM-to-ROM when finishing a commisioning.

Would also like to know more about this whole thing.

Hey Jesper,

Thanks for the response. Could that issue with retentivity have to do with the startup mode? During cold startup the retentive values are erased but if there were a way to set the PLC to startup warm after insertion of the card then it should retain the values, as long as load Memory is an mmc. Not sure if thats even possible, but just a guess...

Jieve said:

Hey Jesper,

Thanks for the response. Could that issue with retentivity have to do with the startup mode? During cold startup the retentive values are erased but if there were a way to set the PLC to startup warm after insertion of the card then it should retain the values, as long as load Memory is an mmc. Not sure if thats even possible, but just a guess...

Click to expand...

Which version of TIA portal You're using, and which version of firmware PLC has ?

JesperMP said:

by doing a RAM-to-ROM when finishing a

Click to expand...

I am very sorry to tangent this thread but i am curious. What is a ram to rom copy? In particular, what function does it serve to help the commisioning engineer or any person that follows?
Again sorry for diverting the thread...

RAM-to-ROM copies all the actual values from work memory to load memory (MMC card). If you do not do this, an eletrical glitch, removal-insertion of the MMC card, etc. may reset the online actual values to the offline actual values from when you downloaded the DBs.

Ok. Thanks.