This paper presents the design of a monitoring and control system that will be installed in buildings and used as a building management system for monitoring dan controlling mechanical and electrical devices embedded in the building. The system implements the master slave RS485 multidrop configuration. The system hardware consists of sensor, controller, and actuator. Arduino board with AT Mega series microcontroller unit (MCU) is used as controller. MCU's ADC will be used as sensor. MODBUS remote terminal unit is used as protocol and implemented inside the master and slave progam inside the MCU. At the end of this paper, the result of transmission with various baud rate setting, various cable length, multiple message frames and are presented.