Siemens SIMATIC S7 Manual page 22

start_x
end_x
internal_send_
buffer
internal_recv_
buffer
MODBUS / TCP communication via the integrated PN interface of the CPU
2XV9450-1MB02; Manual edition 2.1
Start_x specifies the first register or bit address, which is stored in the data
element 0 of the DB. End_x defines the of the last MODBUS address.
When accessing registers, the number of the data element of the S7 DB in
which the last register is mapped, can be calculated with the following
formula:
DBW number = (end_x – start_x) ∗ 2
When accessing coils or inputs, the number of the data element of the S7 DB
in which the last bit is mapped, can be calculated with the following formula:
DBB number = (end_x – start_x +7) / 8
The defined memory areas must not overlap. The parameter end_x must not
be smaller than start_x. In case of an error occurring, the initialization of the
FB is stopped with an error. When start_x is equal to end_x, one Modbus
address (1 register or 1 bit) is allocated.
In section 6.3 you can find an example of the mapping of the MODBUS
addresses to S7 memory areas.
start_x, end_x
MODBUS address
This array is used internally for message data within the FB. Accessing or
changing the array is inadmissible.
This array is used internally for the received data within the FB. Accessing or
changing the array is inadmissible.
Parameterisation of the Modbus Communication
0 to 65535 (W#16#0000 to W#16#FFFF)
4-8