Melody Interface Specification

Melody exists as a series of cartridges for Atari XE/XL
Each cartridge has several sound chips with common access interface, and audio input and output for easy daisy-chaining one to the other. For this purpose a passive cartridge expander was built 

Currently there are 5 Melody cartridges
PSG - contains 2 PSG chips
TurboSound - contains 2 Yamaha OPN chips, and a single SAA1099 chip
OPX - contains Yamaha OPL(2 or 3) chip and Yamaha OPM chip
OPN2 - contains 2 Yamaha OPN2 chips and SN76489A chip
SID - contains 2 SID chips - either MOS6581 or MOS8580

All registers are located in cartridge space, to minimise memory footprint this is done as follows:

Address:
    0xD5DF    -    Melody interface ID/Address register
        Reads    - 0x4D (Ascii “M”)
        Writes    -    any value from 0x00 to 0xFF is treated as bus address
            if device is present on given address it will respond with activating its registers at following addresses:
    0xD5D8 - 0xD5DA    -    Device ID
        Currently defined:
        PSG        -    address 0x50
        OPL        -    address 0x4C
        OPN        -    address 0x4E
        ON2        -    address 0x32
        OPM        -    address 0x$D
        SAA        -    address 0x53
        SN7        -    address 0x37
        SID        -    address 0x49
        reserved    -    0x00 (all devices are off the bus)

    0xD5DB    -    Interface version (4.4bit, binary) (cartridges are 0.8)
    0xD5DC    -    Device Base address (device dependent resolution)
    0xD5DE    -    Device Control
        bit    0    SCL
        bit    1    SDA
        bit    6    Clock source (device dependent)
        bit    7    Device reset (default 1 - device in reset state)
SCL and SDA provides access to SDA and SCL lines of MCP4651 - dual digital potentiometer controlling output volume level for easy sound mixing of multiple devices
Due to the simplistic hardware, all I2C communication is done at 6502 side in software
this I2C interface is present in Melody bus starting with version 0.8