Siemens SIMATIC S7-200 System Manual page 457

Data Consistency
PROFIBUS supports three types of data
consistency:
Byte consistency ensures that
-
bytes are transferred as whole
units.
Word consistency ensures that
-
word transfers cannot be
interrupted by other processes in
the CPU (the two bytes composing
the word are always moved
together and cannot be split). Use
Word consistency if the data
values being transferred are
integers. integers.
Buffer consistency ensures that the entire buffer of data is transferred as a single unit,
-
uninterrupted by any other process in the CPU. Buffer consistency should be used if the
data values are double words or floating point values or when a group of values all relate to
one calculation or item.
You set the data consistency as part of the I/O configuration in the master. The data consistency
selection is written to the DP slave as part of the initialization of the slave. Both the DP master and
the DP slave use the data consistency selection to be sure that data values (bytes, words, or
buffers) are transferred uninterrupted within master and slave. The different types of consistency
are shown in Figure A-29.
User Program Considerations
Once the EM 277 PROFIBUS--DP module has been successfully configured by a DP master, the
EM 277 and the DP master enter data exchange mode. In data exchange mode, the master writes
output data to the EM 277 PROFIBUS--DP module, the EM 277 module then responds with most
current S7-200 CPU input data. The EM 277 module continuously updates its inputs from the
S7-200 CPU in order to provide the most recent input data to the DP Master. The module then
transfers the output data to the S7-200 CPU. The output data from the master is placed into V
memory (the output buffer) starting at the address that the DP master supplied during initialization.
The input data to the master is taken from the V memory locations (the input buffer) immediately
following the output data.
The output data from the master must be moved by the user program in the S7-200 CPU from the
output buffer to the data areas where it is to be used. Likewise, the input data to the master must
be moved from the various data areas to the input buffer for transfer to the master.
Output data from the DP master is placed into V memory immediately after the user program
portion of the scan has been executed. Input data (to the master) is copied from V memory to the
EM 277 for transfer to the master at the same time.
Output data from the master is only written into V memory when there is new data available from
the master.
Input data to the master are transmitted to the master on the next data exchange with the master.
The starting address of the data buffers in V memory and the size of the buffers must be known at
the time the user program for the S7-200 CPU is created.
Technical Specifications
Master
Byte 0
Byte 1
Byte 2
Byte 3
Byte 0
Byte 1
Byte 2
Byte 3
Byte 0
Byte 1
Byte 2
Byte 3
Byte 4
Byte 5
Byte 6
Byte 7
Figure A-29 Byte, Word, and Buffer Data Consistency
Appendix A
Slave
Byte 0
Byte 1
Byte consistency
Byte 2
Byte 3
Byte 0
Byte 1
Word consistency
Byte 2
Byte 3
Byte 0
Byte 1
Byte 2
Byte 3
Buffer consistency
Byte 4
Byte 5
Byte 6
Byte 7
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