This application note describes a multimaster field-bus protocol, using a power line modem based on the ST7540 (refer to application note AN2451) and an STM32 microcontroller used for interfacing the modem. This protocol is used for managing data exchange between different mains-connected devices. Possible applications are power management, heating or cooling system management, lighting control, alarm systems, and remote control appliances. The protocol implements a multimaster type, where each device can be a master in a communication starting the transmission. This introduces the need of data collision management.