Siemens SIMATIC S7 System Manual page 193

The arrows indicate how each array is associated to different MB_MASTER instructions.
The first element of any array or structure is always the first source or destination of any
Modbus read or write activity. All the scenarios below are based on the diagram above.
Scenario 1: If the first MB_MASTER instruction reads 3 words of data from Modbus
address 40001 on any valid Modbus slave, then the following occurs.
The word from address 40001 is stored in "Data".Array_1[1].
The word from address 40002 is stored in "Data".Array_1[2].
The word from address 40003 is stored in "Data".Array_1[3].
Scenario 2: If the first MB_MASTER instruction reads 4 words of data from Modbus
address 40015 on any valid Modbus slave, then the following occurs.
The word from address 40015 is stored in "Data".Array_1[1].
The word from address 40016 is stored in "Data".Array_1[2].
The word from address 40017 is stored in "Data".Array_1[3].
The word from address 40018 is stored in "Data".Array_1[4].
S7-1200 Programmable controller
System Manual, 11/2009, A5E02486680-02
Programming instructions
6.3 Global library instructions
193