Siemens SIMATIC S7-400 Manual page 79

Isochronous Updating of Process Image Partitions
SFC126 "SYNC_PI" is used for the isochronous update of the process image partition of
inputs. An application program which is interconnected to a DP cycle can use the SFC for
consistent updates of the data recorded in the process image partition of inputs in
synchronism with this cycle. SFC126 accepts interrupt control and can only be called in the
OBs 61, 62, 63 and 64.
SFC 127 "SYNC_PO" is used for the isochronous update of the process image partition of
outputs. An application program which is interconnected to a DP cycle can use the SFC for
the consistent transfer of the computed output data of a process image partition of outputs to
the I/O in synchronism with this cycle. SFC127 accepts interrupt control and can only be
called in the OBs 61, 62, 63 and 64.
To allow isochronous updates of process image partitions, all input or output addresses of a
slave must be assigned to the same process image partition.
To ensure consistency of data in a process image partition, the following conditions must be
satisfied on the various CPUs:
● CPU 414: Number of slaves + number of bytes / 100 < 26
● CPU 416: Number of slaves + number of bytes / 100 < 40
The SFCs 126 and 127 are described in the corresponding Online Help and in the
System and Standard Functions
"
Consistent User Data
Data that belongs together in terms of its content and describes a process state at a specific
point in time is known as consistent data. To maintain consistency, the data should not be
changed or updated during processing or transmission.
You will find a detailed description of this procedure in Subsection
Sync/Freeze
The SYNC control command is used to set sync mode on the DP slaves of selected groups.
In other words, the DP master transfers current output data and instructs the relevant DP
slaves to freeze their outputs. The DP slaves writes the output data of the next output frames
to an internal buffer; the state of the outputs remains unchanged.
Following each SYNC control command, the DP slaves of the selected groups transfer the
output data stored in the internal buffer to the process outputs.
The outputs are only updated cyclically again after you transfer the UNSYNC control
command using SFC11 "DPSYC_FR".
The FREEZE control command is used to set the relevant DP slaves to Freeze mode, in
other words, the DP master instructs the DP slaves to freeze the current state of the inputs.
It then transfers the frozen data to the input area of the CPU.
Following each FREEZE control command, the DP slaves freeze the state of their inputs
again.
The DP master receives the current state of the inputs cyclically again not until you have
sent the UNFREEZE control command with SFC11 "DPSYC_FR".
For information on SFC11, refer to the corresponding online help and to the
Standard Functions
S7-400 Automation System, CPU Specifications
Manual, 10/2006, 6ES7498-8AA04-8BA0
" manual.
manual
5.1 CPU 41x-3 PN/DP as DP master / DP slave
Consistent User Data
PROFIBUS DP
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System and
5-5