Consistent Data Access Without The Use Of Sfc 14 Or Sfc 15 - Siemens SIMATIC S7-400H Manual

4.10.5

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 listed below. The data area of a DP slave that should transfer
consistently is transferred to a process image partition. The information in this area
are therefore always consistent. You can subsequently access the process image
partition using the load / transfer command (e.g. L EW 1). This is an especially
convenient and efficient (low runtime load) way to access consistent data. It
enables efficient connection and configuration of such devices as drives or other
DP slaves.
This applies to CPUs 41x-H with firmware version 3.0 or later:
No I/O access errors occur with direct access (e.g. L PEW or T PAW).
The following is important for converting from the SFC14/15 method to 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.
• SFC 50 "RD_LGADR" outputs another address area with the SFC 14/15
method as with the process image method.
• If you are using a CP 443-5 ext the simultaneous use of system functions 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.
Example:
The following example (of the process image partition 3 "TPA 3") shows such a
configuration in HW Config:
• TPA 3 at output: These 50 bytes are stored consistent in the process image
partition 3 (pull–down list "Consistent over –> entire length") and can therefore
be read through the normal "load input xy" commands.
• Selecting "Process Image Partition –> –––" under input in the pull–down menu
means: do not store in a process image. Then the handling can only be
performed using the system functions SFC14/15.
Automation System S7-400H Fault-tolerant Systems
A5E00068197-07
Installation of a CPU 41x-H
4-47