Siemens Simatic S7-200 System Manual page 176

Network Communications and the S7-200 CPU
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. This means that the two bytes composing the word are always moved
together and cannot be split.
Buffer consistency ensures that the entire buffer of data is transferred as a single unit,
uninterrupted by any other process in the CPU.
Word and buffer consistency force the CPU to halt any other processes, such as user
interrupts, while manipulating or moving the DP I/O data within the CPU. Word consistency
should be used if the data values being transferred are integers. Buffer consistency should
be used if the data values are double words or floating point values. Buffer consistency
should also be used 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.
Figure 9-11 shows the different types of consistency.
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 9-11
9-20
Byte, Word, and Buffer Data Consistency
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
S7-200 Programmable Controller System Manual
C79000-G7076-C230-02