Introduction #

The MELBUS (Mitsubishi Electric Bus) is a proprietary communication protocol developed by Mitsubishi Electric, primarily used in automotive infotainment systems. It was widely implemented in OEM radios, CD changers, amplifiers, and navigation units supplied to manufacturers such as Volvo.

Unlike CAN or LIN, MELBUS is not a general-purpose vehicle network. Instead, it is a specialized serial bus designed for low-speed, command–response communication between a head unit (master) and peripheral devices (slaves). Its purpose is to provide a reliable and cost-effective link for audio control and data exchange in car multimedia systems.

Some key characteristics of MELBUS include:

• Master–slave architecture – The head unit is always the master; peripherals such as CD changers, MD players, or navigation modules act as slaves.
• Low data rate – Sufficient for control commands, metadata, and basic diagnostics (not for streaming audio).
• Dedicated wiring – Typically three lines: clock, data, and enable, separate from vehicle-wide buses like CAN.
• Proprietary protocol – Frame format, message definitions, and command sets are manufacturer-specific and not openly standardized.
• Integration with vehicle systems – In some applications (e.g., Volvo), the MELBUS head unit also interfaces with CAN for overall system coordination.

Because of these characteristics, MELBUS was a practical solution for modular multimedia expansion before the widespread adoption of MOST, Ethernet, and Bluetooth-based infotainment networks.

Protocols #

Since MELBUS is proprietary, much of its detail is manufacturer-specific. At a high level, its communication can be described in two layers:

MELBUS Physical Layer & Signaling #

MELBUS typically uses:

• Synchronous serial transmission – With a dedicated clock line generated by the master.
• One master, multiple slaves – Only one device responds at a time, selected by the enable line.
• Simple framing – Data is transmitted in small packets (frames) containing control commands or status responses.
• Polling mechanism – The head unit polls attached devices to check for presence and status.

This ensures predictable, low-overhead communication suitable for audio system control.

Control & Diagnostic Messaging #

On top of the physical signaling, MELBUS defines a set of control messages for device interaction. Common functions include:

• Device control – Play, stop, next/previous track, eject, etc.
• Status reporting – Disc loaded, track number, playback state, error conditions.
• Metadata exchange – Limited transfer of text information such as CD track numbers or radio station IDs.
• Diagnostics – Basic fault signaling (e.g., “device not responding”) handled at the head unit.

In some implementations, extensions allow MELBUS devices to interoperate with other diagnostic frameworks (for example, CAN or UDS gateways in later Volvo platforms).

Together, MELBUS provided a straightforward, manufacturer-controlled way to expand audio and navigation functionality, bridging the gap between early analog control systems and later high-speed digital multimedia networks.