With the rising complexity and processing power of modern computer systems, the amount of MCU on a single PCB also rises. These microcontrollers often need to communicate with each other to exchange payload and control information in a bidirectional manner. Today’s well-established communication protocols in MCUs either do not fit modern transmission speed requirements or do have an inappropriate master-slave attribute, which does not allow the communication partners to have equal bus access rights. Therefore, this paper introduces an extension of the Serial Peripheral Interface (SPI) to allow an equally distributed access right for the communication interface between two microcontrollers. It simultaneously does fit modern transmission speed requirements of a common network interface, so that the message transmission does not constitute a bottleneck in data processing. Besides the protocol design, we do also provide a first prototype implementation, which constitutes a proof of concept.