Siemens SIMATIC S7-1200 System Manual page 1184

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Communication processor and Modbus TCP

13.5 Modbus communication

HR_Start_Offset

Modbus holding register addresses begin at 40001. These addresses correspond to the

beginning PLC memory address of the holding register. However, you can use the

"HR_Start_Offset" variable to start the beginning Modbus holding register address at another

number instead of 40001.

For example, if the holding register starts at MW100 and is 100 words long. An offset of 20

specifies a beginning holding register address of 40021 instead of 40001. Any address less

than 40021 or greater than 40119 results in an addressing error.

Table 13- 68 Example of Modbus holding register addressing

HR_Start_Offset

HR_Start_Offset is word data in the MB_SERVER instance data block that assigns the

starting address of the Modbus holding register. You can set this public static variable by

using the parameter helper drop list, after MB_SERVER is placed in your program.

For example, after you place MB_SERVER in a LAD network, you can go to a previous

network and assign HR_Start_Offset. The start address must be assigned prior to execution

of MB_SERVER.

Modbus address (Word)

S7-1200 address

Modbus address (Word)

S7-1200 address

Entering a Modbus server

variable using the default DB name:

1. Set the cursor in the parameter field and

type an m character.

2. Select "MB_SERVER_DB" from the drop

list of DB names.

"MB_SERVER_DB.HR_Start_Offset" from

the drop list of DB variables.

System Manual, V4.2, 09/2016, A5E02486680-AK

S7-1200 Programmable controller