8 pin DIN female connector on the CD Changer.
The Alpine M-Bus interface is a proprietary single-wire, half-duplex serial bus running at approximately 10 V idle, used to connect compatible CD changers to Alpine head units via an 8-pin DIN connector. All known protocol details are reverse-engineered, as Alpine has never published an official M-Bus specification; signalling uses pulse-width modulation with logical 0 encoded as roughly 0.6 ms pulses and logical 1 as roughly 1.8 ms pulses. The interface was used in Alpine head units such as the CDM-9801 and CDM-9803, and in Acura and Honda OEM head units during the late 1990s and 2000s. Notably, Acura and Honda OEM units swap pins 2 and 4 relative to the Alpine standard pinout, requiring an aftermarket adapter (such as the ACUALP) to use a standard Alpine changer in an Acura vehicle.
| Alpine Pin | Acura Pin | Name | Function | Wire (ext) | Wire (int) |
|---|---|---|---|---|---|
| 8 | 8 | GND | Ground | black | orange |
| 6 | 6 | BATT | +12V | yellow | blue |
| 7 | 7 | ACC | Accessory | red | white |
| 1 | 1 | BUS | Bus | green | red |
| 3 | 3 | B.G | Bus Ground | black | orange |
| 4 | 2 | R | Right Signal | red | blue |
| 2 | 4 | S.G | Signal Ground | black | white |
| 5 | 5 | L | Left Signal | white | red |
| Shell | Shell | Chassis | Chassis Ground | black | Chassis |
Signals
- GND — chassis ground return; shared between Alpine and Acura pin 8.
- BATT — permanent +12 V supply from the vehicle battery; keeps the changer’s memory and disc mechanism powered when ACC is off.
- ACC — switched +12 V accessory supply; powers the changer’s active circuitry when the ignition is on.
- BUS — the M-Bus single-wire serial data line. Idle state is approximately 10 V (head unit pulls the line HIGH via an internal resistor); active-low signalling is achieved via an NPN transistor on the changer side. Packets are nibble-encoded (4-bit hex) with address, command, and an XOR+1 checksum nibble.
- B.G — bus ground reference for the M-Bus data line.
- R — right analogue audio signal (RCA-level); note that Alpine pin 4 corresponds to Acura pin 2.
- S.G — signal ground for the analogue audio channels; note that Alpine pin 2 corresponds to Acura pin 4.
- L — left analogue audio signal (RCA-level).
- Shell — connector shell tied to chassis ground.
Notes
- The pin numbering difference between Alpine and Acura/Honda OEM head units is a critical compatibility issue: pins 2 (S.G) and 4 (R) are swapped. Using a standard Alpine changer directly in an Acura without an adapter cable such as the ACUALP will result in incorrect audio routing.
- Alpine produced two changer bus systems in this era: the older 8-pin DIN M-Bus (used by the CHM-S630 and Acura/Honda OEM units) and the newer Ai-NET proprietary protocol. The KCA-130B cable bridges M-Bus to Ai-NET, allowing Ai-NET changers such as the CHA-1214 to connect to an M-Bus head unit via the adapter. The KCM-122B cable converts M-Bus to RCA for use as an auxiliary input.
- The bus idle voltage of approximately 10 V requires level-shifting circuitry when interfacing with 5 V microcontrollers or other logic; the bus is not TTL-compatible directly.
- When AUX ON mode is active on the head unit, the CD changer on the same M-Bus port is disabled.