Communication processor and Modbus TCP
13.9 Legacy Modbus RTU communication (CM/CB 1241 only)
S7-1200 Programmable controller
System Manual, V4.2, 09/2016, A5E02486680-AK
1275
DATA_ADDR and MODE parameters select the Modbus function type
DATA_ADDR (starting Modbus address in the slave): Specifies the starting address of the
data to be accessed in the Modbus slave.
The MB_MASTER instruction uses a MODE input rather than a Function Code input. The
combination of MODE and Modbus address determine the Function Code that is used in the
actual Modbus message. The following table shows the correspondence between parameter
MODE, Modbus function code, and Modbus address range.
Table 13- 149 Modbus functions
0 01 1 to 2000
1
Read output bits:
1 to (1992 or 2000) bits per request
1 to 9999
0 02 1 to 2000
1 to 1992
1
Read input bits:
1 to (1992 or 2000) bits per request
10001 to 19999
0 03 1 to 125
1
Read Holding registers:
1 to (124 or 125) words per request
40001 to 49999 or
0 04 1 to 125
1
Read input words:
1 to (124 or 125) words per request
30001 to 39999
1 05 1 Write one output bit:
1 to 9999
1 06 1 Write one holding register:
40001 to 49999 or
1 15 2 to 1968
1
Write multiple output bits:
2 to (1960 or 1968) bits per request
1 to 9999
1 16 2 to 123
1
Write multiple holding registers:
2 to (122 or 123) words per request
40001 to 49999 or
2 15 1 to 1968
1
Write one or more output bits:
1 to (1960 or 1968) bits per request
1 to 9999
2 16 1 to 123
1
Write one or more holding registers:
1 to (122 or 123) words per request
40001 to 49999 or
11 11 0 Read the slave communication status word and event
counter. The status word indicates busy (0 – not busy,
0xFFFF - busy). The event counter is incremented for
each successful completion of a message.
Both the DATA_ADDR and DATA_LEN operands of
MB_MASTER are ignored for this function.
80 08 1 Check slave status using data diagnostic code 0x0000
(Loopback test – slave echoes the request)
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