Modbus Rtu - Siemens SIMATIC S7 System Manual

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Network 2: Modbus function 3 - Read holding register words

Modbus RTU

There are two versions of the Modbus RTU instructions available in STEP 7:

● Version 1 was initially available in STEP 7 Basic V10.5.

● Version 2 is available in STEP 7 Basic/Professional V11. The version 2 design adds REQ

For compatibility and ease of migration, you can choose which instruction version to insert

into your user program.

Do not use both 1.x and 2.y instruction versions in the same CPU program. Your program's

Modbus instructions must have the same major version number (1.x, 2.y, or V.z). The

individual instructions within a major version group may have different minor versions (1.x).

When you use the instruction tree to place a Modbus instruction in your program, a new FB

instance is created in the project tree. You can see new FB instance in the project tree under

PLC_x > Program blocks > System blocks > Program resources.

To verify the version of a Modbus instruction in a program, you must inspect project tree

properties and not the properties of a box displayed in the program editor. Select a project

tree Modbus FB instance, right-click, select "Properties", and select the "Information" page to

see the Modbus instruction version number.

S7-1200 Programmable controller

System Manual, 11/2011, A5E02486680-05

and DONE parameters to MB_COMM_LOAD. Also, the MB_ADDR parameter for

MB_MASTER and MB_SLAVE now allows a UInt value for extended addressing.

Click the icon on the instruction tree task card to enable the headers and columns

of the instruction tree.

105BCommunication processor

12.5 Modbus communication

To change the version of the Modbus

instructions, select the version from the drop-

down list. You can select the group or

individual instructions.

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Simatic s7-1200