Siemens SIMATIC S7-1200 - Page 1255

Communication processor and Modbus TCP
13.8 Legacy Modbus TCP communication
S7-1200 Programmable controller
System Manual, V4.2, 09/2016, A5E02486680-AK
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Table 13- 132 MB_CLIENT instance data block user accessible static variables
Variable
Data type
Default
description
Blocked_Proc_Timeout Real 3.0 Amount of time (in seconds) to wait upon a blocked Modbus client
instance before removing this instance as being ACTIVE. This can
occur, for example, when a client request has been issued and then
application stops executing the client function before it has completely
finished the request. The maximum S7-1200 limit is 55 seconds.
MB_Unit_ID Word 255 Modbus unit identifier:
A Modbus TCP server is addressed using its IP address. As a result,
the MB_UNIT_ID parameter is not used for Modbus TCP addressing.
The MB_UNIT_ID parameter corresponds to the slave address in the
Modbus RTU protocol. If a Modbus TCP server is used for a gateway to
a Modbus RTU protocol, the MB_UNIT_ID can be used to identify the
slave device connected on the serial network. The MB_UNIT_ID would
be used to forward the request to the correct Modbus RTU slave ad-
dress.
Some Modbus TCP devices may require the MB_UNIT_ID parameter to
be initialized within a restricted range of values.
RCV_TIMEOUT Real 2.0 Time in seconds that t
he MB_CLIENT waits for a server to respond to a
request.
Connected Bool 0 Indicates whether the connection to the assigned server is connected
or disconnected: 1=connected, 0=disconnected
Table 13- 133 MB_CLIENT protocol errors
STATUS
(W#16#)
Response code to
Modbus client
(B#16#)
Modbus protocol errors
8381
01
Function code not supported
8382
03
Data length error
8383 02 Data address error or access outside the bounds of the MB_HOLD_REG address area
8384
03
Data value error
8385 03 Data diagnostic code value not supported (function code 08)