Siemens SIMATIC S7-1200 System Manual page 1172

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Communication processor and Modbus TCP

13.5 Modbus communication

Parameter and type

CPU firmware version requirement

The Modbus TCP instructions described in this section of the manual require firmware

release V4.1 or later.

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 starts. If the connection is not already established, then a new connection is

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

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.

It is important that the inputs are not changed during the period 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.

0 - No MB_CLIENT operation in progress

1 - MB_CLIENT operation in progress

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.

Execution condition code

S7-1200 Programmable controller

System Manual, V4.2, 09/2016, A5E02486680-AK