Modbus Rtu - Siemens S7-1200 System Manual
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- Operating instructions manual (132 pages) ,
- Programming manualline (74 pages) ,
- Manual (46 pages)
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Communication processor and Modbus TCP
12.5 Modbus communication
Network 2: Modbus function 3 - Read holding register words
12.5.3
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
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.
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.
746
Click the icon on the instruction tree task card to enable the headers and columns
of the instruction tree.
To change the version of the Modbus
instructions, select the version from the drop-
down list. You can select the group or
individual instructions.
S7-1200 Programmable controller
System Manual, 03/2014, A5E02486680-AG
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