Siemens SIMATIC S7-400 Manual page 120

Consistent Data
7.3 Consistent Reading and Writing of Data from and to DP Standard Slaves/IO Devices
For information on SFC 15, refer to the corresponding online help and to the
Standard Functions
Note
The PROFIBUS DP standard defines the upper limit for the transmission of consistent user
data. 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.
Upper Limit for the Transmission of Consistent User Data to a IO Device
There is a 255 bytes upper limit for the transmission of consistent user data on an IO device.
Even when more than 255 bytes can be transmitted on an IO device, only a maximum of 255
bytes can be consistently transmitted.
Consistent Data Access without the Use of SFC 14 or SFC 15
Consistent data access of > 4 bytes without using SFC 14 or SFC 15 is possible for the
CPUs described in this manual. The data area of a DP slave or IO devices that should
transfer consistently is transferred to a process image partition. The information in this area
is therefore always consistent. You can subsequently use load/transfer commands (such as
L IW 1) to access the process image. This is an especially convenient and efficient (low
runtime load) way to access consistent data. This allows efficient integration and
configuration of drives or other DP slaves, for example.
An I/O access error does not occur with direct access (e.g. L PIW or T PQW).
The following is important for converting from the SFC14/15 method to the process image
method:
● SFC 50 "RD_LGADR" outputs another address area with the SFC 14/15 method as with
● PROFIBUS DP via Interface interface:
● PROFIBUS-DP via CP 443-5 Extended:
7-4
manual.
the process image method.
When converting from the SFC14/15 method to the process image method, it is not
recommended to use the system functions and the process image at the same time.
Although the process image is updated when writing with the system function SFC15, this
is not the case when reading. In other words, the consistency between the process image
values and the values of the system function SFC14 is not ensured.
If you are using a CP 443-5 ext, the simultaneous use of SFC14/15 and the process
image results in the following errors, you cannot read/write into the process image and/or
you can no longer read/write with SFC 14/15.
System and
S7-400 Automation System, CPU Specifications
Manual, 10/2006, 6ES7498-8AA04-8BA0