Siemens SIMATIC S7 System Manual page 585

Parameter and type
MB_MODE IN
MB_DATA_ADDR IN
MB_DATA_LEN IN
MB_DATA_PTR IN_OUT
DONE OUT
BUSY OUT
ERROR OUT
STATUS OUT
REQ parameter
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 will start. If the connection is not already established then a new connection
will be 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 successive 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.
As a result, it is important that the inputs are not changed during the period of time that a
MB_CLIENT operation is actively being processed. If this rule is not followed, then a
MB_CLIENT cannot determine that it is the active instance.
S7-1200 Programmable controller
System Manual, 11/2011, A5E02486680-05
Data type Description
USInt Mode Selection: Assigns the type of request (read, write, or diagnostic). See
the Modbus functions table below for details.
UDInt Modbus starting Address: Assigns the starting address of the data to be
accessed by MB_CLIENT. See the Modbus functions table below for valid
addresses.
UInt Modbus data Length: Assigns the number of bits or words to be accessed in
this request. See the Modbus functions table below for valid lengths
Variant Pointer to the Modbus data register: The register buffers data going to or
coming from a Modbus server. The pointer must assign a standard global
DB or a M memory address.
Bool The DONE bit is TRUE for one scan, after the last request was completed
with no error.
Bool
0 - No MB_CLIENT operation in progress

1 - MB_CLIENT operation in progress

Bool The ERROR bit is TRUE for one scan, after the MB_CLIENT execution was
terminated with an error. The error code value at the STATUS parameter is
valid only during the single cycle where ERROR = TRUE.
Word Execution condition code
105BCommunication processor
12.5 Modbus communication
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