Hitachi manufactures takumi series of ductable air conditioners (higher capacity from 3tr to 16.5tr). maximum 16 units of indoor + outdoor can be interconnected thus reducing the field wiring, making it safe installation. as it has got proprietary rs485 protocol, so it cannot be connected to building management system (bms) employed in large complexes like hotels, shopping malls, multiplexes, large offices, etc. since it has no connectivity with the bms, many of the orders were cancelled. bms has rs232 protocol which is again a proprietary protocol. hence for the connectivity between the two we need an open standard protocol i.e., modbus open rtu protocol. modbus is an application layer messaging protocol for client/server communication between devices connected on different types of buses or networks. this is intelligent converter from proprietary rs485 protocol to modbus open rtu over rs232 protocol. this converter should be able to get running parameters data from max. 16 indoor units & 16 outdoor units & store the parameters values in memory. on request from modbus master, it should return back parameters values by modbus over rs232. this project can be used anywhere where we are using rs485 protocol for data communication and also can be used anywhere where we are using modbus protocol. and this can be use in many fields of the electronics – like plant automation, etc. this has many advantage as modbus rtu is an open standard protocol making it interoperable with almost any bms in the world. this project can be used commercially.