Skip to main content

    Tab: Modbus TCP Server Device – General

    The tab contains the basic settings for network communication in the network.

    Table 23. Configured Parameters

    Watchdog

    standard icon: Activates the watchdog function.

    The incoming data (holding registers / %I range) is set to zero when the Modbus device does not receive any valid query from the client.

    Closing the TCP socket

    _cds_icon_checked.png: If the watchdog is enabled and the set time is exceeded, then the TCP socket will be closed.

    Server port

    Port number of the server

    Bind to Adapter

    This option is relevant for systems with multiple Ethernet adapters.

    standard icon: The server socket of the Modbus Server is bound to the IP address of the assigned Ethernet adapter. Only Modbus requests which have been sent to this IP address are received.

    _cds_icon_option_deactivated.png: The server socket of the Modbus Server is not bound to an IP address. All Modbus requests are processed, which have been received by the system (on the Modbus port).

    Holding registers (%IW)

    Number of holding registers: possible values: 1–4096. The maximum number can be limited in the device description.

    Writable: standard icon: For the holding register, writable I/O mappings (%QW addresses) are generated instead of read-only I/O mappings (%IW addresses). This allows the holding registers to be set by the Modbus device application (= server application) by means of the usual I/O mapping.

    Input registers (%QW)

    Number of input registers: possible values: 1–4096. The maximum number can be limited in the device description.

    Discrete Bit Areas

    The Modbus coils and discrete inputs get their own data area. By default, the register access and bit access use the same Modbus data.

    Coils

    Number of Modbus coils

    Discrete inputs

    Number of Modbus discrete inputs



    Important

    Changes to the input/output assembly size have a direct effect on the number of configurable parameters in the I/O mapping.

    Table 24. Data Model

    Modbus defines for I/O data a data model with four areas.

    • Coils: Outputs of type BIT

    • Discrete inputs: Inputs of type BYTE

    • Holding register: Outputs of type WORD (16-bit)

    • Input register: Inputs of type WORD (16-bit)

    Inputs and outputs are included as with all fieldbuses, but from the point of view of the "host". These data areas have any number of logical start offsets as defined by the manufacturer. The I/O data behind the Modbus data areas can overlap or be independent of each other.

    For details about the Modbus data model, see Modbus Protocol Specification in Chapters 4.3 and 4.4 (www.modbus.org).

    In the case of the CODESYS Modbus Device, the input registers and discrete inputs, as well as the holding registers and coils, are all mapped to the same I/O data. This is also the case when they are addressed via different logical start offsets.

    Coils

    Definition of the start address of the respective Modbus data area

    Discrete inputs

    Holding register

    Input register

    Holding and input register data areas overlay

    standard icon: Overlay of the process image by the holding and input register.

    This is required, for example, when the server application is used to write to the holding register.