Consistent Data Access Without Using Sfc 14 Or Sfc 15 - Siemens SIMATIC S7-400H System Manual

S7-400H in PROFIBUS DP mode
7.2 Consistent Data
7.2.5

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 which is to be transferred consistently will be written 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 represents a
highly user-friendly and efficient (low runtime load) method to access consistent data and to
implement and configure such devices as drives or other DP slaves.
Any direct access to a data area which is configured consistent,
such as L PEW or T PAW, does not result in an I/O access error.
Important aspects in the conversion from the SFC 14/15 solution to the process image
solution are:
● When converting from the SFC 14/15 method to the process image method, it is not
● SFC 50 "RD_LGADR" outputs another address area with the SFC 14/15 method as with
● When using a CP 443-5 ext, the parallel use of system functions and of the process
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advisable 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 SFC 15, this is not
the case when reading. In other words, consistency between the values of the process
image and of the system function SFC 14 is not ensured.
the process image method.
image leads to 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.
System Manual, 09/2007, A5E00267695-03
S7-400H