Data Consistency; Byte, Word, And Buffer Data Consistency - Siemens SIMATIC S7-200 System Manual

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 A-30 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 A-30
S7-200 Programmable Controller System Manual
A5E00066097-02

Byte, Word, and Buffer Data Consistency

S7-200 Specifications
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
A-57