Siemens S7-1200 System Manual page 756

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Communication processor and Modbus TCP

12.5 Modbus communication

Parameter and type

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

Table 12- 75 Mapping of Modbus addresses to the process image

Read words Input

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 MB_SLAVE instruction.

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

Modbus functions

Address range

New Data Ready:

0 – No new data

1 – Indicates that new data has been written by the Modbus

0 – No data read

1 – Indicates that data has been read by the Modbus master

The ERROR bit is TRUE for one scan, after the last request was

terminated with an error. If execution is terminated with an error,

then the error code value at the STATUS parameter is valid only

during the single scan where ERROR = TRUE.

Execution error code

Output Process Image

Input Process Image

Input Process Image

Output Process Image

Output Process Image

Q0.0 to Q1023.7

I0.0 to I1023.7

IW0 to IW1022

Q0.0 to Q1023.7

S7-1200 Programmable controller

System Manual, 03/2014, A5E02486680-AG