Siemens SIMATIC S7 System Manual page 49

Retentive memory
Data loss after power failure can be avoided by marking certain data as retentive. The
following data can be configured to be retentive:
● Bit memory(M): You can define the precise width of the memory for bit memory in the
● Tags of a function block (FB): If an FB was created with the "Symbolic access only" box
● Tags of a global data block: The behavior of a global DB with regard to retentive state
A total of 2048 bytes of data can be retentive. To see how much is available, from the PLC
tag table or the assignment list, click on the "Retain" toolbar icon. Although this is where the
retentive range is specified for M memory, the second row indicates the total remaining
memory available for M and DB combined.
Diagnostics buffer
The CPU supports a diagnostic buffer which contains an entry for each diagnostic event.
Each entry includes a date and time the event occurred, an event category, and an event
description. The entries are displayed in chronological order with the most recent event at
the top. While the CPU maintains power, up to 50 most recent events are available in this
log. When the log is full, a new event replaces the oldest event in the log. When power is
lost, the ten most recent events are saved.
S7-1200 Programmable controller
System Manual, 11/2009, A5E02486680-02
PLC tag table or in the assignment list. Retentive bit memory always starts at MB0 and
runs consecutively up through a specified number of bytes. Specify this value from the
PLC tag table or in the assignment list by clicking the "Retain" toolbar icon. Enter the
number of M bytes to retain starting at MB0.
selected, then the interface editor for this FB includes a "Retain" column. In this column,
you can select either "Retain" or "Non-Retain" individually for each tag. An instance DB
that was created when this FB is placed in the program editor shows this retain column
as well, but only for display; you cannot change the retentive state from within the
instance DB interface editor for an FB that was configured to be "Symbolic access only".
If an FB was created with the "Symbolic access only" box deselected, then the interface
editor for this FB does not include a "Retain" column. An instance DB created when this
FB is inserted in the program editor shows a "Retain" column which is available for edit.
In this case, selecting the "Retain" option for any tag results in all tags being selected.
Similarly, deselecting the option for any tag results in all tags being deselected. For an FB
that was configured to not be "Symbolic access only", you can change the retentive state
from within the instance DB editor, but all tags are set to the same retentive state
together.
After you create the FB, you cannot change the option for "symbolic access only". This
option can only be selected when the FB is created. To determine whether an existing FB
was configured for "symbolic access only", right-click the FB in the Project tree, select
"Properties", and then select "Attributes".
assignment is similar to that of an FB. Depending on the setting for symbolic addressing
you can define the retentive state either for individual or for all tags of a global data block.
– If the "Symbolic access only" attribute of the DB is checked, the retentive state can be
set for each individual tag.
– If the "Symbolic access only" attribute of the DB is un-checked, the retentive-state
setting applies to all tags of the DB; either all tags are retentive or no tag is retentive.
PLC concepts
3.1 Execution of the user program
49