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Pinout:
Ok the dreaded MELBUS.
After days of research and poking around with logic analyzer here is what we have. There are three wires as seen on the pinout DATA, CLOCK, and BUSY. Lets start with the easy part.
BUSY line is only used for devices to announce themselves on the bus. The line is pulled low for 1 second as soon as the device gets power. The HU in my case HU-613 then proceeds to use the CLOCK and DATA line for any further communication.
CLOCK is provided by the HU for communication to and from devices so there is nothing really to do on the clock line except to listen to the cycles and use them accordingly. Whenever the HU wants to say something on the bus the CLOCK starts ticking for 8 cycles until it is done sending the information. However if the HU is expecting a response it will keep cycling the clock for 8 cycles for each byte that it expects in return without doing anything on the DATA line.
DATA line is read by the devices like CD-Changer on a rising clock pulse. So at the beginning of a byte that the HU sends clock goes from HIGH to LOW and then the device reads the DATA line on every RISING clock edge. After the last bit the CLOCK remains HIGH until the next byte of information. Then the sequence repeats.
Init Sequence
So lets go through the basic init sequence that happens when a CD-Changer gets power.
- The CD-C pulls the BUSY low for 1 second
- The HU responds by sending 3 Init bytes (0x07, 0x1A, 0xEE).
- Then HU proceeds sending device ids followed by an empty 0xFF byte each.
- The CD-C waits until it reads its id on the bus (0xE8) and fills the next byte with the same byte 0xE8
- The HU keeps going through the rest of ids in case there are other devices on the bus
After Init the HU then addresses each device for basic information preceding each request with a unit id of the device it is talking to. In case of the CD-Changer the first byte will be 0xE8 and then the request. 0x1E 0xEF is the request that comes in for the CD-Changer asking it to provide CD cartridge info. Then it keeps cycling the clock until it got the information from the CD-C unit. Then the bus goes idle and the CD-CHANGER appears on the screen of the HU. No more communication goes on until CD-Changer is selected as source on the HU.
