Also giva a look at this instructables page https://www.instructables.com/Arduino-for-a-Wired-Home-Assistant-Network/
Arduino sketch for modbus implementation of sensor, switch and thermostat to use with Home Assistant. This code make the arduino (I've used an a 5V nano) act a Modbus device than can be connected to a Home Assistant controller. Connectione can be made via a usb cable or via 485 bus. Only in this mode you can connect more than a device and with long cables (more than 1000m). Home Assistant deals with 4 types of device:
This arduino can be 1 or all of these object: it can be a temperature sensor if a sensor is connected. Can be a Climate (sensor also needed). Can read up to 16 inputs and control up to 16 outputs (this arduino have less this number of pins). The temperature read by sensor is mapped in the current_temp_register that can be read by HA. Inputs and outputs are mapped to binary_inputs register and binary_outputs register respectively.
See electical schematic for details.
This hardware can serve to many goals: it can be a headless remote actuator (a switch) to control a load (a lihgt, a motor, etc) it can be a headless remote sensor for temperature reading of to check magnetic sensors it can have a display and buttons to show temperature or control lighs or other devices
Modbus registers map All registers are 16 bits wide
reg# reg name function 0 TARGET_TEMP_REGISTER_LOW setpoint temperature 1 TARGET_TEMP_REGISTER_HIGH 2 CURRENT_TEMP_REGISTER_LOW current tempperature read by sensor 3 CURRENT_TEMP_REGISTER_HIGH 4 BINARY_INPUTS read the inputs connected (read only) 5 BINARY_OUTPUTS read and set the outputs connected 6 BINARY_OUTPUTS_LIGHTS read and set the outputs connected (a light output is also controlled by a pin connected to a switch button)