Communication processor and Modbus TCP
13.5 Modbus communication
S7-1200 Programmable controller
1172 System Manual, V4.2, 09/2016, A5E02486680-AK
BUSY OUT Bool
• 0 - No MB_CLIENT operation in progress
• 1 - MB_CLIENT operation in progress
ERROR OUT Bool The ERROR bit is TRUE for one scan, after the MB_CLIENT execution
ended with an error. The error code at the STATUS parameter is valid
only during the single cycle where ERROR = TRUE.
Note
CPU firmware version requirement
The Modbus
TCP instructions described in this section of the manual require firmware
FALSE = No Modbus communication request
TRUE = Request to communicate with a Modbus TCP server
If no instance of MB_CLIENT is active and parameter DISCONNECT=0, when REQ=1 a new
Modbus request starts. If the connection is not already established, then a new connection is
made.
If the same instance of MB_CLIENT is executed again with DISCONNECT=0 and REQ=1,
before the completion of the current request, then no subsequent Modbus transmission will
be made. However, as soon as the current request is completed, a new request can be
processed if MB_CLIENT is executed with REQ=1.
When the current MB_CLIENT communication request is complete, the DONE bit is TRUE
for one cycle. The DONE bit can be used as a time gate to sequence multiple MB_CLIENT
requests.
Note
Input data consistency during MB_CLIENT processing
Once a Modbus client initiates a Modbus operation, all the input states are saved internally
and are then compared on each succ
essive call. The comparison is used to determine if this
particular call was the originator of the active client request. More than one MB_CLIENT call
can be performed using a common instance DB.
It is important that the inputs are not changed during the p
eriod of time that an MB_CLIENT
operation is actively being processed. If this rule is not followed, then an MB_CLIENT cannot
determine the active instance.
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