Siemens S7-1200 System Manual page 753

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For "Extended Addressing" mode the maximum data lengths are reduced by 1 byte or 1 word depending upon the data

type used by the function.

DATA_PTR parameter

The DATA_PTR parameter points to the DB or M address that is written to or read from. If

you use a data block, then you must create a global data block that provides data storage for

reads and writes to Modbus slaves.

The DATA_PTR data block type must allow direct addressing

The data block must allow both direct (absolute) and symbolic addressing. When you create

the data block the "Standard" access attribute must be selected.

Data block structures for the DATA_PTR parameter

● These data types are valid for word reads of Modbus addresses 30001 to 39999, 40001

● For bit reads and writes of Modbus addresses 00001 to 09999 and bit reads of 10001 to

● Although not required, it is recommended that each MB_MASTER instruction have its

● There is no requirement that the DATA_PTR data areas be in the same global data block.

S7-1200 Programmable controller

System Manual, 03/2014, A5E02486680-AG

Operation and data

Reset slave event counter using data diagnostic code

1 word per request

to 49999, and 400001 to 465536 and also for word writes to Modbus addresses 40001 to

49999 and 400001 to 465536.

– Standard array of WORD, UINT, or INT data types

– Named WORD, UINT, or INT structure where each element has a unique name and

16 bit data type.

– Named complex structure where each element has a unique name and a 16 or 32 bit

– Standard array of Boolean data types.

– Named Boolean structure of uniquely named Boolean variables.

own separate memory area. The reason for this recommendation is that there is a greater

possibility of data corruption if multiple MB_MASTER instructions are reading and writing

to the same memory area.

You can create one data block with multiple areas for Modbus reads, one data block for

Modbus writes, or one data block for each slave station.

Communication processor and Modbus TCP

12.5 Modbus communication