Siemens SIMATIC S7 Manual page 21

connect_at_
startup
8 Data Areas
data_type_x
db_x
MODBUS / TCP communication via the integrated PN interface of the CPU
2XV9450-1MB02; Manual edition 2.1
This parameter defines the point in time at which the connection is being
established.
FALSE: establish connection when ENQ_ENR=TRUE
TRUE: establish connection immediate after PLC start-up
When connect_at_startup = TRUE, the first request should be started after
the connection was established (CONN_ESTABLISHED = TRUE) or an error
has been reported respectively (ERROR =TRUE).
8 data areas are available to map the Modbus addresses in the S7 storage.
At least the first data area has to be defined. The other 7 data areas are
optional. Depending on the type of request, these memory areas are read or
written.
With one request, only one DB can be accessed. Even if consecutive register
numbers or coils are located in two different DBs, two requests are necessary
to access them both. This has to be taken into account during the
parameterization.
It is possible to map more Modbus values (registers or coils) in one data block
than could be processed with one telegram.
The parameter data_type_x defines the MODBUS data type which is to be
mapped with this definition.
If data_type_x set to 0, this data area is not used
Identifier Data type
0 Area not used
1 Coils
2 Inputs
3 Holding Register
4 Input Register
The parameter db_x defines the number of the data block in which the
consecutively defined MODBUS registers or bits are mapped.
0 cannot be used as DB number since it is reserved for system functions.
db_x
DB-Number
Parameterisation of the Modbus Communication
Size
Bit
Bit
Word
Word
1 - 65535 (W#16#0001 to W#16#FFFF)
4-7