Consistent Data
7.3 Consistent Reading and Writing of Data from and to DP Standard Slaves/IO Devices
S7-400 Automation System, CPU Specifications
Manual, 10/2006, 6ES7498-8AA04-8BA0
7-3
7.3 Consistent Reading and Writing of Data from and to DP Standard
Slaves/IO Devices
Reading Data Consistently from a DP Standard Slave/IO Device Using SFC 14 "DPRD_DAT"
Using SFC14 "DPRD_DAT" (read consistent data of a DP standard slave) you can
consistently read the data of a DP standard slave.
If no error occurred during the data transmission, the read data are entered in the destination
area defined by RECORD.
The destination area must be the same length as the one you configured for the selected
module with
STEP 7
.
By invoking SFC14 you can only access the data of one module / DP ID at the configured
start address.
For information on SFC14, refer to the corresponding online help and to the
System and
Standard Functions
manual
Writing Data Consistently to a DP Standard Slave/IO Device 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 or IO device addressed in the RECORD.
The source area must be the same length as the one you configured for the selected module
with
STEP 7
.
Upper Limit for the Transmission of Consistent User Data to a DP Slave
The PROFIBUS DP standard defines the upper limit for the transmission 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 slave.
During the configuration you can determine the size of the consistent area. You can set a
maximum length of consistent data at 64 words = 128 bytes in the special identification
format (SKF) (128 bytes for inputs and 128 bytes for outputs); the data block size cannot
exceed this.
This upper limit only applies to pure user data. Diagnostics and parameter data are
regrouped into full records and therefore always transferred consistently.
In the general identification format (AKF) the maximum length of consistent data can be set
at 16 words = 32 bytes (32 bytes for inputs and 32 bytes for outputs); the data block size
cannot exceed this.
Note in this context that a CPU 41x in a general environment acting as a DP slave on a third-
party master (connection defined by GSD) has to be configured with the general
identification format. The transfer memory of a CPU 41x acting as a DP slave to the
PROFIBUS DP can therefore be a maximum of 16 words = 32 bytes.
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