Siemens SIMATIC S7-1200 Manual page 19

3 Mode of Operation
With read access, the read data are successively ("RegisterIndex") written into the
corresponding register file ("Index") of the addressed fan depending on the access
mode:

Interface parameter "holding" for the holding registers ("mode" = 0)

Interface parameter "input" for the input registers ("mode" = 104)
If necessary, the read value ("uint") is subsequently converted into the
corresponding floating point value ("real") depending on the calculation instruction
("calc").
With write access ("write.req"), the entered floating point value ("real") is converted
into the corresponding data value in the format "UInt" if necessary and
subsequently written into the data buffer "dataPtr" of the Modbus_Master and
transmitted to the fans ("write.fanRange.start" bis "write.fanRange.end") based on
the calculation instruction ("calc").
As the only exception, the holding register "Password" to write to has a length of
3 words (see
elements are transmitted ("dataPtr [0..2] = uint [0..2]").
Controlling several fans (ebm-papst) via Modbus RTU
Entry ID: 109476801, V2.0,
Table 3-1). Here, the first and additionally the two following field
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