hai friends...
i am completely beginner for modbus.. my arduino uno will act as master and read the float value by rs485 in slave id 2.
i dont know what to do.. please help me foe this
ModBus is a serial protocol to read sensor (slave) values by a master. It has functions to set/get bit and integer values. There are several implementations (libraries) for ModBus on the Arduino, many with advantages, all with some disadvantages.
As the physical connection of your ModBus device uses the standard RS-485 you first have to get the required hardware for that. That hardware usually builds upon a MAX485 or some similar chip. There are cheap Chinese modules available on ebay and amazon but these often pose problems as the quality is not the best and the designers had only single node applications in mind (shouldn't be a problem in your case).
As ModBus RTU runs on a serial interface the baudrate and the other configuration parameters may be different from device to device. There is a default in the ModBus standard but many devices don't care for that. Depending on these parameters not all Arduino models are useful for that task.
Please post a link to the manual of the device you want to attach to your Arduino, so we get the necessary information to help you further.