Address Mapping - Siemens SIMATIC S7 Manual

WRITE_READ
5.3

Address Mapping

Interpretation of
Modbus
Addresses
OPEN MODBUS / TCP communication via CP343-1 and 443-1
2XV9450-1MB00; Manual edition 4.3
This parameter defines if a read or write function should be carried out. If the
value of the input/output is FALSE, it specifies the read mode. The value
TRUE specifies the write mode.
Only Holding Registers and Coils can be written. Input Registers and Inputs
can be read merely.
In the operating mode "CP is client" it is an input parameter. In the operating
mode "CP is server" it is an output parameter.
MODBUS bases its data model on a series of tables, which have
distinguishing characteristics. The distinction between these memory areas is
done via the register address by some systems, e.g. MODICON PLCs. So a
MODBUS message requesting the read of a holding register at offset 0 would
return the value known to the application programmer as found in register
40001 (memory type 4xxxx, reference 0001).
One potential source of confusion is the varying interpretation of the register
address in different manuals. Sometimes the register address means the
address of the application layer, sometimes the address transferred.
The FB MODBUS uses the Modbus address transferred at its parameters
start_x und START_ADDRESS. So it is possible to use register addresses
from von 0000 to FFFF
H
with each function code.
H
Function Block MODBUSCP
5-12