Datablocks in Siemens S7 PLC's


Thread Starter

Mols, Jo [JanBe]

Hi list,

In a recently installed PLC (S7-300 CPU 315-2dp) several setpoints for controllers, alarmvalues, max. speedvalues, ... are stored in datablocks in the Actual value part. The initial value is not filled in. What happens during a restart (or after voltagedrop)? Are the initials copied to the actuals (so we loose all current setpoints ??) ? Is there a danger that we loose the actuals ? Is it necessary to have the same initial and actual value ???

Tx in advance,

The actual values are the values used on startup of the PLC. To prevent a loss of the actuals upload the DB's. It is not necessary to have the same Initial and actual value.


Jacek Dobrowolski

From: "Dobrowolski, Jacek" <[email protected]>
To: [email protected]
Subject: RE: PLCS: Datablocks in Siemens S7 PLC's


Initial values overwrite your actual values when the CPU memory reset is performed, you perform program download including the datablocks, the CPU
starts up and the CPU BATF led is on.


Jacek Dobrowolski, M. Sc. E. Eng.
Software Eng.

Donald Pittendrigh

Hi All

Avccording to the Siemens manual and also to my experience of what really happens, the only signifoicance of the Initial value is that it is the value assigned to the data when editing the DB for the first time, I dont think the memory reset CPu restart and other CPU related matters have any bearing on the matter at all and if you are sure of this I would like to know if you have seen it happen.

The initial value is seen in the fifth column when editing a DB, it is there to initialise the DB if an initial data value is not entered manually.

In the case of an instance DB or a DB tied to a specific UDT, the initial value will be written into the data when re-generating the DB for whatever reason.

See the Siemens manual "Statement list for S7300 and S7400 programming section 3.3.

Donald Pittendrigh