Step 7 shared data block and M

[larry]

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In S7 400H ，The device's turn signal is saved to the M store, which is used in other fail-safe block programs, but why not save the turn signal to a shared data block ? What are the advantages of M storage over data blocks?

 

[L4s3r73k]

Hello Larry.

1:
Some use to say, that the M-Storage of actual CPUs of Siemens is just some emulated stuff for not having to say to old programmers, that this does not exist anymore. So there are some documents and voices from Siemens I've heard that they recommend not to use anymore this M-Datafields and to use global datablocks instead.

2:
To do some interchanging of data between the normal program and the failsafe program it is recommended to have a coupling global datablock. This is to not refere directly so safe data in normal program and unsafe data in the safe program. There is some Siemens documentation somewhere saying the same.

This is my concern.