Communication processor and Modbus TCP
13.5 Modbus communication
S7-1200 Programmable controller
System Manual, V4.2, 09/2016, A5E02486680-AK
1207
Modbus communication function codes (1, 2, 4, 5, and 15) can read and write bits and words
directly in the input process image and output process image of the CPU. For these function
codes, the MB_HOLD_REG parameter must be defined as a data type larger than a byte.
The following table shows the example mapping of Modbus addresses to the process image
in the CPU.
Table 13- 82 Mapping of Modbus addresses to the process image
01 Read bits Output 1 to 8192 Output Process Image Q0.0 to Q1023.7
Modbus communication function codes (3, 6, 16) use 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 on the Modbus_Slave instruction.
Note
MB_HOLD_REG data block type
A Modbus holding register data block must allow
both direct (absolute) and symbolic
addressing. When you create the data block the "Standard" access attribute must be
selected.
The following table shows examples of Modbus address to holding register mapping that is
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 13- 83 Mapping of Modbus addresses to CPU memory
MB_HOLD_REG parameter examples
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