Siemens SIMATIC S7-1500 Function Manual page 87

Open User Communication
6.7 Setting up communication with Modbus TCP
Modbus TCP server as gateway to Modbus RTU
If you use a Modbus TCP server as a gateway to a Modbus RTU protocol, address the slave
device in the serial network using the static parameter, MB_UNIT_ID. The MB_UNIT_ID
parameter corresponds to the field of the slave address in the Modbus RTU protocol. The
MB_UNIT_ID parameter in this case would forward the request to the correct Modbus RTU
slave address.
You do not have to program the gateway function yourself.
You can find the MB_UNIT_ID parameter in the instance data block associated with
MB_CLIENT instruction.
You can find more information on the MB_UNIT_ID parameter in the STEP 7 online help.
Reference
● This FAQ (https://support.industry.siemens.com/cs/ww/en/view/94766380) describes how
to program and configure the Modbus TCP communication between two S7-1500 CPUs.
● This FAQ (https://support.industry.siemens.com/cs/ww/en/view/102020340) describes
how to program and configure Modbus TCP communication between an S7-1500 CPU
and an S7-1200 CPU.
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Function Manual, 11/2019, A5E03735815-AH
Communication