Open User Communication
6.7 Setting up communication with Modbus TCP
Communication
132
Function Manual, 05/2021, A5E03735815-AJ
Redundant communication over Modbus TCP
Assign parameters for redundant communication via Modbus TCP using the MB_RED_CLIENT
or MB_RED_SERVER instructions:
Instruction MB_RED_CLIENT You use the instruction "MB_RED_CLIENT" to establish a
redundant connection between the client and the server, send Modbus requests, receive
responses and control connection termination of the Modbus TCP client.
Instruction MB_RED_SERVER The "MB_RED_SERVER" instruction processes connection
requests of a Modbus TCP client, receives and processes Modbus requests and sends
responses. The CPUs can:
•
processing multiple server connections and
•
accepting multiple connections from different clients simultaneously at one server port.
For more information on the MB_RED_CLIENT or MB_RED_SERVER instructions, refer to the
STEP 7 online help.
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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