Siemens SIMATIC S7-400H System Manual page 114

Consistent data
9.3 Consistent reading and writing of data from and to DP standard slaves/IO devices
Writing data consistently to a DP standard slave using SFC 15 "DPWR_DAT"
Using SFC 15 "DPWR_DAT" (write consistent data to a DP standard slave), you transmit the
data in RECORD consistently to the addressed DP standard slave or IO device.
The source area must have the same length as the one you configured for the selected
module with STEP 7.
For information on SFC 15, refer to the corresponding Online Help and to the "System and
Standard Functions" manual.
Note
When a send operation is activated (positive edge at REQ), the data to be transmitted from
the send areas SD_i is copied from the user program. You can write new data to these areas
after the block call command without corrupting the current send data.
Upper limits for the transfer of consistent user data to a DP slave
The PROFIBUS DP standard defines upper limits for the transfer of consistent user data to a
DP slave. For this reason a maximum of 64 words = 128 bytes of user data can be
consistently transferred in a block to the DP standard slave.
You can define the length of the consistent area in your configuration. In the special
identification format (SIF), you can define a maximum length of consistent data of 64 words =
128 bytes; 128 bytes for inputs and 128 bytes for outputs. A greater length is not possible.
This upper limit applies only to pure user data. Diagnostics and parameter data are grouped
to form complete data records, and are thus always transferred consistently.
In the general identification format (GIF), you can define a maximum length of consistent
data of 16 words = 32 bytes; 32 bytes for inputs, and 32 bytes for outputs. A greater length is
not possible.
In this context, consider that a CPU 41x operating as DP slave generally has to support its
configuration at an external master (implementation by means of GSD file) using the general
identification format. A CPU 41x operated as DP slave thus supports only a maximum length
of 16 words = 32 bytes in its transfer memory for PROFIBUS DP.
Note
The PROFIBUS DP standard defines the upper limits for transmission of consistent user
data. Typical DP standard slaves adhere to this upper limit. Older CPUs (<1999) had CPU-
specific restrictions in terms of the transmission of consistent user data. The maximum
length of data this CPU can consistently read and write to and from a DP standard slave is
specified in your technical specifications, keyword "DP Master – User data per DP slave".
With this value, newer CPUs exceed the length of data that a DP standard slave provides or
receives.
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System Manual, 03/2012, A5E00267695-11
S7-400H