Modbus is a common industrial field control bus protocol which is not only used in a wide variety of industrial applications like SCADA systems but also commonly used in building management systems(BMS) across the globe.Buildings generally use BTU,Water,Energy meters that operate on M-bus standard.M-bus devices cannot communicate directly with a Modbus based Building Management System(BMS).The network level incompatibility of a M-bus device to communicate with a Modbus master can be overcome by using a protocol conversion device having low conversion delay time and suitable voltage adjustment circuits.My project will focus on building a protocol conversion gateway for the M-bus slaves to communicate with a Modbus based master with minimum time delay using RS232 based serial medium for communication.