Siemens SIMATIC S7-400H System Manual page 115

Upper limits of the length of consistent user data transmitted to an IO Device
The length of consistent user data that you can transmit to an IO device is limited to 1025
bytes (= 1024 bytes user data + 1 byte secondary value). Irrespective of whether you can
transmit more than 1024 bytes to an IO device, the transmission of consistent data is still
limited to 1024 bytes.
When operating in PN-IO mode, the length of data transmission via CP 443-1 is limited to
240 bytes.
Consistent data access without using SFC 14 or SFC 15
Consistent data access > 4 bytes is also possible without using SFC 14 or SFC 15. The data
area of a DP slave or IO device to be transmitted consistently is transferred to a process
image partition. The data in this area are thus always consistent. You can then access the
process image partition using the load/transfer commands (L EW 1, for example). This
provides a particularly convenient and efficient (low runtime load) method of accessing
consistent data. Thus an efficient integration and parameterization of, for example, drives or
other DP slaves is made possible.
Any direct access to a consistently configured data area,
e.g. L PEW or T PAW, does not result in an I/O area access error.
Important aspects in the conversion from the SFC14/15 solution to the process image
solution are:
● SFC 50 "RD_LGADR" outputs different address areas with the SFC 14/15 method as with
● PROFIBUS DP via internal interface:
● PROFIBUS DP via CP 443-5 ext:
S7-400H
System Manual, 03/2012, A5E00267695-11
9.3 Consistent reading and writing of data from and to DP standard slaves/IO devices
the process image method.
When converting from the SFC14/15 method to the process image method, it is not
advisable to use the system functions and the process image concurrently. Although the
process image is updated when writing with system function SFC15, this is not the case
when reading. In other words, consistency between the values of the process image and
of the system function SFC14 is not ensured.
If you use a CP 443–5 ext, the parallel use of system functions and process image
causes the following errors: Read/write access to the process image is blocked, and/or
SFC 14/15 is no longer able to perform any read/write access operations.
Note
Forcing variables
It is not allowed to force variables in the I/O or process image range of a DP slave or IO
device and that belong to a consistency range. The user program may overwrite these
variables in spite of the force job.
Consistent data
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