esp-idf/components/freemodbus/docs/en/port_initialization.rst

.. _modbus_api_port_initialization:

Modbus Port Initialization
^^^^^^^^^^^^^^^^^^^^^^^^^^

The ESP_Modbus supports Modbus SERIAL and TCP ports and a port must be initialized before calling any other Modbus API. The functions below are used to create and then initialize Modbus controller interface (either master or slave) over a particular transmission medium (either Serial or TCP/IP):

- :cpp:func:`mbc_slave_init`
- :cpp:func:`mbc_master_init`
- :cpp:func:`mbc_slave_init_tcp`
- :cpp:func:`mbc_master_init_tcp`

The API call uses the first parameter to recognize the type of port being initialized. Supported enumeration for different ports: :cpp:enumerator:`MB_PORT_SERIAL_MASTER`, :cpp:enumerator:`MB_PORT_SERIAL_SLAVE` accordingly. 
The parameters :cpp:enumerator:`MB_PORT_TCP_MASTER`, :cpp:enumerator:`MB_PORT_TCP_SLAVE` are reserved for internal usage.

.. code:: c

    void* master_handler = NULL; // Pointer to allocate interface structure
    // Initialization of Modbus master for serial port
    esp_err_t err = mbc_master_init(MB_PORT_SERIAL_MASTER, &master_handler);
    if (master_handler == NULL || err != ESP_OK) {
        ESP_LOGE(TAG, "mb controller initialization fail.");
    }

This example code to initialize slave port:

.. code:: c

    void* slave_handler = NULL; // Pointer to allocate interface structure
    // Initialization of Modbus slave for TCP 
    esp_err_t err = mbc_slave_init_tcp(&slave_handler);
    if (slave_handler == NULL || err != ESP_OK) {
        // Error handling is performed here
        ESP_LOGE(TAG, "mb controller initialization fail.");
    }