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Communication processor and Modbus TCP

13.8 Legacy Modbus TCP communication

MB_MODE and MB_DATA_ADDR parameters select the Modbus communication function

MB_DATA_ADDR assigns the starting Modbus address of the data to be accessed. The

MB_CLIENT instruction uses a MB_MODE input rather than a function code input.

The combination of MB_MODE and MB_DATA_ADDR values determine the function code

that is used in the actual Modbus message. The following table shows the correspondence

between parameter MB_MODE, Modbus function, and Modbus address range.

Table 13- 131 Modbus functions

Operation and data

Read output bits:

1 to 2000 bits per request

Read input bits:

1 to 2000 bits per request

Read Holding registers:

1 to 125 words per request

Read input words:

1 to 125 words per request

Write one output bit:

One bit per request

Write one holding register:

1 word per request

Write multiple output bits:

2 to 1968 bits per request

Write multiple holding registers:

2 to 123 words per request

Write one or more output bits:

1 to 1968 bits per request

Write one or more holding registers:

1 to 123 words per request

Read the server communication status word

and event counter. The status word indi-

cates busy (0 – not busy, 0xFFFF - busy).

The event counter is incremented for each

successful completion of a message.

Both the MB_DATA_ADDR and

MB_DATA_LEN parameters of MB_CLIENT

are ignored for this function.

Check server status using data diagnostic

code 0x0000 (Loopback test – server ech-

oes the request)

1 word per request

MB_DATA_ADDR

10001 to 19999

40001 to 49999 or

400001 to 465535

30001 to 39999

40001 to 49999 or

400001 to 465535

40001 to 49999 or

400001 to 465535

40001 to 49999 or

400001 to 465535

S7-1200 Programmable controller

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

Siemens SIMATIC S7-1200 System Manual page 1252

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