Siemens SIMATIC S7 System Manual page 591
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MB_SERVER allows incoming Modbus function codes (1, 2, 4, 5, and 15) to read or write
bits and words directly in the input process image and output process image of the S7-1200
CPU. For data transfer function codes (3, 6, and 16), the MB_HOLD_REG parameter must
be defined as a data type larger than a byte. The following table shows the mapping of
Modbus addresses to the process image in the CPU.
Table 12- 57 Mapping of Modbus addresses to the process image
Codes
Function
01
Read bits
02
Read bits
04
Read words Input
05
Write bit
15
Write bits
Incoming Modbus message function codes function codes (3, 6, and 16) read or write words
in a Modbus holding register which can be an M memory address range or a data block. The
type of holding register is specified by the MB_HOLD_REG parameter.
Note
MB_HOLD_REG parameter assignment
The Modbus Holding Register can be in a standard global DB or a M memory address.
For A Modbus holding register in M memory, use the standard Any Pointer format. This is in
the format P#"Bit Address" "Data Type" "Length". An example would be P#M1000.0 WORD
500
The following table shows examples of Modbus address to holding register mapping used for
Modbus function codes 03 (read words), 06 (write word), and 16 (write words). The actual
upper limit of DB addresses is determined by the maximum work memory limit and M
memory limit, for each CPU model.
Table 12- 58 Mapping examples of Modbus address to CPU memory address
Modbus Address
40001
40002
40003
40004
40005
Multiple server connections
Multiple server connections may be created. This permits a single PLC to establish
concurrent connections to multiple Modbus TCP clients.
S7-1200 Programmable controller
System Manual, 11/2011, A5E02486680-05
Modbus functions
Data area
Address range
Output
1
Input
10001
30001
Output
1
Output
1
P#M100.0 Word 5
MW100
MW102
MW104
MW106
MW108
Data area
To
8192
Output Process Image
To
18192
Input Process Image
To
30512
Input Process Image
To
8192
Output Process Image
To
8192
Output Process Image
MB_HOLD_REG parameter examples
P#DB10.DBx0.0 Word 5
DB10.DBW0
DB10.DBW2
DB10.DBW4
DB10.DBW6
DB10.DBW8
105BCommunication processor
12.5 Modbus communication
S7-1200
CPU address
Q0.0 to Q1023.7
I0.0 to I1023.7
IW0 to IW1022
Q0.0 to Q1023.7
Q0.0 to Q1023.7
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591
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