Legacy Modbus Rtu Communication (Cm/Cb 1241 Only); Overview; Selecting The Version Of The Modbus Rtu Instructions - Siemens SIMATIC S7-1200 System Manual

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Legacy Modbus RTU communication (CM/CB 1241 only)

Prior to the release of STEP 7 V13 SP1 and the S7-1200 V4.1 CPUs, the Modbus RTU

communication instructions existed with different names, and in some cases, slightly

different interfaces. The general concepts apply to both sets of instructions. Refer to the

individual legacy Modbus RTU instructions for programming information.

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

● Legacy version 1.3: Compatible with all CPU and CP versions

● Legacy version 2.2: Compatible with all CPU and CP versions

(Note: The version 2.2 design adds REQ 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.)

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

into your user program.

You cannot use both versions of the instructions with the same module, but two different

modules can use different versions of the instructions. Your program's Modbus RTU

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

13.9 Legacy Modbus RTU communication (CM/CB 1241 only)

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

of the instruction tree.

Communication processor and Modbus TCP

To change the version of the Modbus in-

structions, select the version from the drop-

down list. You can select the group or indi-

vidual instructions.