Siemens SIMATIC S7-400H Manual page 85

4.10.4 Reading Data consistently from a DP Standard Slave and
Writing Consistently to a DP Standard Slave
Reading Data Consistently from a DP Standard Slave Using SFC 14
"DPRD_DAT"
Using SFC 14 "DPRD_DAT" (read consistent data of a DP standard slave) you can
consistently read the data of a DP standard slave.
The data read is entered into the destination range defined by RECORD if no error
occurs during the data transmission.
The destination range must have the same length as the one you have configured
for the selected module with STEP 7.
By invoking SFC 14 you can only access the data of one module / DP ID at the
configured start address.
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
can consistently write data to the DP standard slave addressed in the RECORD.
The source range must have the same length as the one you have configured for
the selected module with STEP 7.
Note
The Profibus DP standard defines the upper limit for the transmission of consistent
user data (see following section). Typical DP standard slaves adhere to this upper
limit. In older CPUs (<1999) there are restrictions in the transmission of consistent
user data depending on the CPU. For these CPUs you can determine the
maximum length of the data which the CPU can consistently read and write to and
from the DP standard in the respective technical specifications under the index
entry "DP Master – User data per DP slave". Newer CPUs are capable of
exceeding the value for the amount of data that a DP standard slave can send and
receive.
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
Installation of a CPU 41x-H
4-45