S7-200 Specifications
A-58
S7-200 Programmable Controller System Manual
A5E00066097-02
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 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. 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 this 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.
SMB200 through SMB249 provide status information about the EM 277
PROFIBUS-DP slave module if it is the first intelligent module in the I/O chain. If
the EM 277 is the second intelligent module in the I/O chain, then the EM 277
status is obtained from SMB250 through SMB299. These SM locations show
default values if DP communication has not been established with a master. After a
master has written parameters and I/O configuration to the EM 277
PROFIBUS-DP module, these SM locations show the configuration set by the DP
master. You should check SMB224 to be sure that the EM 277 is currently in data
exchange mode with the master before using the information in SMB225 through
SMB229 (see Table A-19), or data in the V memory buffer.
Note
You cannot configure the EM 277 PROFIBUS-DP I/O buffer sizes or buffer
location by writing to memory locations SMB225 through SMB229 or SMB275
through SMB279. Only the DP master can configure the EM 277 PROFIBUS-DP
module for DP operation.
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