Siemens Simatic S7-200 System Manual page 390

S7-200 Programmable Controller 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 (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.
-
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-23.
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
A
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.
Status Information
There are 50 bytes of special memory (SM) allocated to each intelligent module based on its physical
position. The module updates the SM locations corresponding to the modules' relative position to the CPU
(with respect to other modules). If it is the first module, it updates SMB200 through SMB249. If it is the
second module, it updates SMB250 through SMB299, and so on. See Table A-39.
378
Master
Byte 0 Byte 0
Byte 1 Byte 1
Byte 2 Byte 2
Byte 3 Byte 3
Byte 0 Byte 0
Byte 1 Byte 1
Byte 2 Byte 2
Byte 3 Byte 3
Byte 0 Byte 0
Byte 1 Byte 1
Byte 2 Byte 2
Byte 3 Byte 3
Byte 4 Byte 4
Byte 5 Byte 5
Byte 6 Byte 6
Byte 7 Byte 7
Figure A-23 Byte, Word, and Buffer Data Consistency
Slave
Byte consistency
Word consistency
Buffer consistency