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Modbus Addresses in Modbus TCP Environment

Many of our Eurortherm devices (MINI 8, 3504, etc.) connect to our Ethernet network. Each Ethernet device has a unique IP address.

We have several pieces of equipment where a non-Ethernet device (example 2000 series) is connected to an Ethernet device (example 3504). Both devices appear in iTools at the same TCP/IP address but with different Modbus addresses.

When multiple Ethernet devices are connected to the network, must each device on the network have a unique Modbus address in this Modbus TCP environment?

Or, does each Ethernet TCP/IP connection create a unique identifier on the Modbus network?

For example, if we have ten Modbus TCP devices connected to the Ethernet and each of them has a unique TCP/IP address, can we assign the same Modbus address to each device (example 10.10.1.10 Modbus 10; 10.10.1.11 Modbus 10) or must each Modbus TCP device have its own Modbus address (example 10.10.1.10 Modbus 10; 10.10.1.11 Modbus 15)?
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  • Jon (Official Rep) February 11, 2019 17:15
    Hi Brad,
    I don't see any problem with having different IP's with the same Modbus address - I'll confirm tomorrow. Jon
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  • Jon,
    I would appreciate clarification. A few months ago I spoke with someone at Eurotherm tech support that advised that each device must have a unique Modbus address. I don't remember who I spoke with.

    They explained that a Modbus Master will identify each device by its Modbus address first and then lookup its TCP/IP address based on the Modbus address.

    Our Modbus devices were configured by a third party.
    We've had some communication issues on our network. Changing each device's Modbus address is on my list of corrective actions.

    Just want to confirm that this will have a positive impact before I change them all.

    The third party provided three control cabinets. Each are similar. The Eurotherm devices in each cabinet are addressed:

    Recorder = Modbus #1
    3504 = Modbus 15
    2000 series = Modbus 16

    Each control cabinet was then connected to our industrial network. The Recorder and 3504 each have a unique IP address. The 2000 series devices are wired via RS232 to the 3504.

    My plan is to provide a unique Modbus address to each Modbus device on the network regardless of its IP address.
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  • Jon (Official Rep) February 12, 2019 12:10
    Hi Brad,
    No problem with regards to the Modbus /TPC devices

    Cabinet #1

    Recorder: IP address XX.YY.ZZ.11 / Modbus Slave address/ID #1
    3504: IP address XX.YY.ZZ.12 / MODBUS Slave address/ID #1

    Which 2000 series controller do you have? The 24xx series only have 1 Serial port and you said this is connected to the 3504 so I don't see how itools would see it. However, if it was connected to the recorder, it supports Modbus/TCP talk-through so itools would see the 2000 unit.

    let me know, Jon
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  • Jon,
    The 2000 series device is the 2204E.
    In one of the cabinets we have

    one 6180A recorder at IP 10.10.2.24
    one 3504 at IP 10.10.2.21
    one 2204E with serial connection
    one PG8-53E2VA

    The Ethernet devices are each connected to a port on an un-managed switch.

    Each cabinet is configured similarly. The Ethernet devices have different IP addresses in each cabinet.

    My question in general is more of a Modbus "best practices" rather than an implementation specific question.
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