S7 - What is in the System Data Block?

Hi All,

I have always believed that the system data block in Siemens Step 7 contained the system config information.

As a test, I created a simple project with just a 315 cpu and 1 input card and 1 output card. I then went online in the blocks folder with a 315 cpu that had a simple rack with a around 6 i/o cards. I then transfered the system data block from the online cpu to my simple project.

I expected the config in my original offline project to then contain the same config as the one that I had uploaded the system data block from but the config still contained just the simple single cpu with 1 input card and 1 output card. (hope that makes sense!). However, when I then compare the 2 system data files they are the same.

So, I guess my question is - what I exactly I have uploaded from the online project that is in the system data block and what effect does over writing the system data block file have?

Thanks

Jack

System data block does contain the configuration (and maybe something else). But this block is not used by the Hardware editor - the projects configuration is kept somewhere else. This block contains *compiled* configuration in a way that the PLC can understand it (remember the "Save and compile" button?), this block is used only by PLC and not the S7 software. If Step7 kept the actual config information in this block how would you change the configuration in an empty project without the CPU and Blocks folder? And try deleting the SDB from the project - you will not lose the configuration.

Of course there is an exception when system data block IS used by Step7 - when uploading a whole station to the PG. Step7 uploads this block, decompiles it and saves the configuration somewhere else. From this point on the config is not loaded from the SDB anymore.

So, I guess my question is - what I exactly I have uploaded from the online project that is in the system data block and what effect does over writing the system data block file have?

Click to expand...

You uploaded the compiled SDB from the online project, the configuration of the online station. But since S7 doesn't load the config from SDB, you don't see any changes. Try "PLC > Upload Station to PG" and the new stations configuration will contain your 6 I/O modules.

You can overwrite and even delete the SDB in the *offline* project, this block can be easily regenerated by opening the hardware editor and clicking on "Save and Compile". But if you delete the SDB in the *online* project (or overwrite it with incorrect configuration) CPU will switch to STOP and show an error. Although I'm not sure if it is going to happen immediately or only when transitioning from STOP to RUN.

Thanks for the reply SFPB, that has cleared a lot of things up for me.

Jack

Hi,

If the HW changes are not saved in SDB, then where are they?

I changed the setting of the CPU in Netpro, for example I add a TCP/IP connection, then compile the project and download it to the PLC. When I compare my old project (include SDB) to the online version, they match perfectly. I know they do not match because of the changes in Netpro.

Why the changes are not shown in the compare function?

Thanks,
Samad