CD drive

CD2 CD drive board.

Or "CD drive". The part that fails before everything else in CD systems. Also see optical pickups.

The NeoGeo CD top-loading model drive was made by Sony. The CDZ one was made by Sharp ?

It is made of:

A 4-bit maskROM MCU (Sony CXP5084), taking care of sequencing and communication (command execution and status report).
A DSP (Sony CXD2500) to process analog data from the pickup.
A servo controler (Sony CXA1372) to drive the pickup coils for tracking and focus.
A spindle and sled motor driver (Rohm BA6396).

Connections

Some say the protocol was standard for the time, but clear information is hard to find. This was guessed from the system ROM disassembly and a logic analyzer.

Connector pinout, directions are from CDD (CD Drive) to CDC (CD Controller):

Pin #	Name	Type	Direction	Description
1	CDCK	Clock	Output	Tells the CDC when to change HOCK
2	?	?	?	High ?
3	/IRQ	Signal	Output	Asks the CDC for a data exchange (low pulse at ~64Hz)
4	?	?	?	High ?
5	HOCK	Clock	Input	Tells the CDD data is ready on D0~3
6	D0~3	Data	Both	4-bit data bus
7
8
9
10	?	?	?	?
11	BCLK?	Clock	Output	I2S bit clock, should be 44100 * 32 = 1.4112MHz
12	SDATA	Data	Output	I2S data
13	LRCK	Signal	Output	I2S word clock, should be 44100Hz. Audio L/R switch for the DAC.
14	MUTE?	Signal	Input	Put low to gate SDATA ?
15	GND	Power	-	Ground
16	5V	Power	-	+5V for logic
17	9V	Power	-	+9V for motors and coils
18	GND	Power	-	Ground

Some signal has to be "D/M", an output which indicates if SDATA is audio or data. There's a /RESET somewhere also.

Communication

NEO-MGA takes care of the I/Os through registers REG_CDDCTRL, REG_CDDINPUT, REG_CDDOUTPUT.

The CDD regularly sends a low pulse on its /IRQ output to initiate a data exchange.
HOCK goes low, indicating that the CDC aknowledges the request. The 4-bit bus direction is CDD to CDC.
CDCK goes low, indicating that the CDD is starting to send its status data (green A). Diagram fix: A should be ready on CDCK low.
HOCK goes high and the CDC reads the status data (A).
CDCK goes returns high.
HOCK goes returns high.
CDCK goes low, the B word is ready.
The cycle continues on for 10 words.

(The delay here is not necessary, but it can be seen on the NeoGeo CD and is caused by the system ROM program preparing stuff).

HOCK goes low and the 4-bit bus direction is inverted (CDC to CDD). The command data is set (red A).
CDCK goes low, indicating that the CDD has read the data (A).
HOCK returns high.
CDCK returns high.
HOCK goes low, the B word is ready.
The cycle continues on for 10 words.

Status data (green)

A: Status
B: Status
C: Minutes high digit
D: Minutes low digit
E: Seconds high digit
F: Seconds low digit
G: Frames high digit
H: Frames low digit
I: Extension
J: Checksum (see below)

Command data (red)

A: Command
B~H: Depends on command
I: Checksum (see below)

Checksum

The checksum digit is the sum of all 9 previous digits, + 5, XOR 15, modulo 15: ((sum + 5) ^ 0xF) & 0xF.

Commands

0: STATUS

No parameters.

1: STOP ALL

No parameters. Stops CDDA playback, disk spin, and sets drive in data mode.

2: TOC COMMAND

D: Command code:

0: GET POSITION

MSF in status will be the current position from the beginning of the CD (absolute).

1: GET TRACK POSITION

MSF in status will be the current position from the beginning of the track (relative).

2: GET TRACK NUMBER

Minutes in status will be the current track.

3: GET CD LENGTH

MSF in status will be the total CD length.

4: GET FIRST AND LAST TRACKS

Minutes in status will be the first track (normally 1), seconds will be the last.

5: GET TRACK NUMBER

MSF in status will be the start of the desired track. If the track is a data track, the frames high digit's fourth bit will be 1 (OR 8).

3: PLAY

B: Minutes high digit
C: Minutes low digit
D: Seconds high digit
E: Seconds low digit
F: Frames high digit
G: Frames low digit

Starts playing audio.

4: SEEK

B: Minutes high digit
C: Minutes low digit
D: Seconds high digit
E: Seconds low digit
F: Frames high digit
G: Frames low digit

Sets pickup at desired position.

6: PAUSE

Pauses CDDA playback (disk keeps spinning).

7: RESUME

Resumes CDDA playback.

8: FAST-FORWARD

Unknown.

9: REWIND