Siemens S7-1200 System Manual page 760

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Communication processor and Modbus TCP

12.5 Modbus communication

For example, after MB_SLAVE is placed in a LAD network, you can go to a previous network

and assign the HR_Start_Offset value. The value must be assigned prior to execution of

Extended_Addressing

The Extended_Addressing variable is accessed in a similar way as the HR_Start_Offset

reference discussed above except that the Extended_Addressing variable is a Boolean

value. The Boolean value must be written by an output coil and not a move box.

Modbus slave addressing can be configured to be either a single byte (which is the Modbus

standard) or double byte. Extended addressing is used to address more than 247 devices

within a single network. Selecting extended addressing allows you to address a maximum of

64000 addresses. A Modbus function 1 frame is shown below as an example.

Table 12- 80 Single-byte slave address (byte 0)

Valid Response

Error response

Table 12- 81 Double-byte slave address (byte 0 and byte 1)

Valid Response

Error response

Slave address

Entering a Modbus slave variable using the

default DB name:

1. Set the cursor in the parameter field and

2. Select "MB_SLAVE_DB" from the drop-list.

3. Set the cursor at the right side of the DB

4. Select "MB_SLAVE_DB.HR_Start_Offset"

Start address

Start address

type an m character.

name (after the quote character) and enter

a period character.

from the drop list.

Length of coils

S7-1200 Programmable controller

System Manual, 03/2014, A5E02486680-AG

Length of coils