Modbus TCP not working across secondary subnet with dual-interface setup

1

Hi everyone,
I’m using OpenHAB with a dual-interface server. The main network is 192.168.1.0/24, and the Modbus devices are on a separate subnet 10.42.42.0/24.
The OpenHAB server has access to both networks — the primary IP is 192.168.1.1.

I’m using the Modbus binding to connect to a device at 10.42.42.2:1502, but I keep getting connection timeouts.


> 2025-04-13 17:48:54.016 [WARN ] [ing.ModbusSlaveConnectionFactoryImpl] - Connect reached max tries 1, throwing last error: Connect timed out. Connection TCPMasterConnection [m_Socket=Socket[unconnected], m_Timeout=3000, m_Connected=false, m_Address=/10.42.42.2, m_Port=1502, m_ModbusTransport=net.wimpi.modbus.io.ModbusTCPTransport@dfd160c, m_ConnectTimeoutMillis=10000, rtuEncoded=false]. Endpoint ModbusIPSlaveEndpoint [address=10.42.42.2, port=1502] 
> 2025-04-13 17:48:54.016 [WARN ] [ing.ModbusSlaveConnectionFactoryImpl] - Error connecting connection TCPMasterConnection [m_Socket=Socket[unconnected], m_Timeout=3000, m_Connected=false, m_Address=/10.42.42.2, m_Port=1502, m_ModbusTransport=net.wimpi.modbus.io.ModbusTCPTransport@dfd160c, m_ConnectTimeoutMillis=10000, rtuEncoded=false] for endpoint ModbusIPSlaveEndpoint [address=10.42.42.2, port=1502]: Connect timed out 
> 2025-04-13 17:48:54.016 [WARN ] [rt.modbus.internal.ModbusManagerImpl] - Could not connect to endpoint ModbusIPSlaveEndpoint [address=10.42.42.2, port=1502] -- aborting request ModbusReadRequestBlueprint [slaveId=71, functionCode=READ_MULTIPLE_REGISTERS, start=56, length=1, maxTries=3] [operation ID 0b227963-c7d8-44f5-b070-026df8cfc5bb]

The Port is open an Ping is working. Is there a way to force the Modbus binding to use the second interface (e.g. via localAddress) or any other idea why the connection might fail?

Thanks in advance!

2

Have you setup both network cards in OpenHAB?

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3

Yes I did setup both network cards as follows:

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do you have any idea on how to give modbus the relevant hint?

The interesting thing is, that the Kostal-Binding is able to connect to the IP 10.42.X.X whereas Modbus TCP can’t.

4

Have you gotten the Modbus to work at all? I would try it on a flat network first to see if it is a network issue or a Modbus issue. Yes Modbus is a standard that some vendors do silly things with .

5

yes the Modbus Binding is working if I’m using the 192.168 interface but not if I’m using the second interface.