In this paper, a new high-speed bus topology LAN protocol for automobiles is proposed and its compatibility with the traditional CAN protocol is discussed. The transfer rate of traditional CAN cannot be increased due to its bitwise arbitration using message priority and Acknowledgment mechanisms. Our proposed protocol avoids collisions by assigning one or more time slots to each ECU within a communication cycle. ACK is informed through the ACK bits included in the header of each frame. Based on this protocol, we implemented a 5 Mbit/s LAN system on which the existing software for CAN is used with small modifications. This paper describes the proposed protocol, named “Scalable CAN,” and its implementation on FPGA, and demonstrates that existing software can be easily ported to this new protocol.