# Modbus TCP not working across secondary subnet with dual-interface setup **URL:** \ **Category:** Off-Topic\ **Tags:** modbus-tcp\ **Created:** [April 13, 2025, 4:06pm UTC](https://community.openhab.org/t/modbus-tcp-not-working-across-secondary-subnet-with-dual-interface-setup/163641 "2025-04-13T16:06:48Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![frieso](https://community.openhab.org/letter_avatar_proxy/v4/letter/f/94ad74/32.png) [@frieso](https://community.openhab.org/u/frieso)\ **Post date:** [April 13, 2025, 4:06pm UTC](https://community.openhab.org/t/modbus-tcp-not-working-across-secondary-subnet-with-dual-interface-setup/163641/1 "2025-04-13T16:06:48Z")
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. ```auto > 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! ---
**Author:** ![denominator](https://community.openhab.org/user_avatar/community.openhab.org/denominator/32/37349_2.png) [@denominator](https://community.openhab.org/u/denominator)\ **Post date:** [April 14, 2025, 1:34pm UTC](https://community.openhab.org/t/modbus-tcp-not-working-across-secondary-subnet-with-dual-interface-setup/163641/2 "2025-04-14T13:34:24Z")
Have you setup both network cards in OpenHAB? ![image](https://community-openhab-org.s3.dualstack.eu-central-1.amazonaws.com/original/3X/a/e/aefa51cc8f2055d032e9e33bfaa0511baf1a8797.png) ---
**Author:** ![frieso](https://community.openhab.org/letter_avatar_proxy/v4/letter/f/94ad74/32.png) [@frieso](https://community.openhab.org/u/frieso)\ **Post date:** [April 14, 2025, 1:42pm UTC](https://community.openhab.org/t/modbus-tcp-not-working-across-secondary-subnet-with-dual-interface-setup/163641/3 "2025-04-14T13:42:15Z")
Yes I did setup both network cards as follows: ![image](https://community-openhab-org.s3.dualstack.eu-central-1.amazonaws.com/original/3X/9/7/97e34e1fde8868026c71055e61a174f15f169e13.png) 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. ---
**Author:** ![denominator](https://community.openhab.org/user_avatar/community.openhab.org/denominator/32/37349_2.png) [@denominator](https://community.openhab.org/u/denominator)\ **Post date:** [April 14, 2025, 2:52pm UTC](https://community.openhab.org/t/modbus-tcp-not-working-across-secondary-subnet-with-dual-interface-setup/163641/4 "2025-04-14T14:52:49Z")
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 . ---
**Author:** ![frieso](https://community.openhab.org/letter_avatar_proxy/v4/letter/f/94ad74/32.png) [@frieso](https://community.openhab.org/u/frieso)\ **Post date:** [April 14, 2025, 3:04pm UTC](https://community.openhab.org/t/modbus-tcp-not-working-across-secondary-subnet-with-dual-interface-setup/163641/5 "2025-04-14T15:04:49Z")
yes the Modbus Binding is working if I’m using the 192.168 interface but not if I’m using the second interface. ---
**Author:** ![Dave\_Baldwin](https://community.openhab.org/user_avatar/community.openhab.org/dave_baldwin/32/45297_2.png) [@Dave\_Baldwin](https://community.openhab.org/u/Dave_Baldwin)\ **Post date:** [May 19, 2025, 2:16pm UTC](https://community.openhab.org/t/modbus-tcp-not-working-across-secondary-subnet-with-dual-interface-setup/163641/6 "2025-05-19T14:16:43Z")
Did you ever get this resolved? Have you verified with a third-party Modbus test client that the Modbus server in question is actually on port 1502, and not 502? (1502 is much less common, but not unheard-of port) ---
**Author:** ![frieso](https://community.openhab.org/letter_avatar_proxy/v4/letter/f/94ad74/32.png) [@frieso](https://community.openhab.org/u/frieso)\ **Post date:** [May 19, 2025, 2:20pm UTC](https://community.openhab.org/t/modbus-tcp-not-working-across-secondary-subnet-with-dual-interface-setup/163641/7 "2025-05-19T14:20:14Z")
Yes I got it working. I removed the Modbus-Binding and reinstalled it. Now it works. ---
**Author:** ![system](https://community-openhab-org.s3-eu-central-1.amazonaws.com/original/2X/6/6a52f2ac5d00e4c7b50471a354b3a9221188be90.png) [@system](https://community.openhab.org/u/system)\ **Post date:** [June 30, 2025, 6:21am UTC](https://community.openhab.org/t/modbus-tcp-not-working-across-secondary-subnet-with-dual-interface-setup/163641/8 "2025-06-30T06:21:08Z")
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