CD drive

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Top-loading drive control board.

The part that fails before everything else in CD systems. Made of a control board and an optical pickup.

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

The control board has:

  • A 4-bit mask ROM MCU (Sony CXP5084), taking care of operation sequencing and communication with the NeoGeo.
  • 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, sled (and tray) motor driver.

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, Sega MegaCD documents and a logic analyzer.

Top-loading model

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

Pin # Name Type Direction Description
1 CDCK Clock Output Communication clock to CDC
2  ?  ?  ? High ?
3 /IRQ Signal Output Asks the CDC for a data exchange (low pulse at ~64Hz)
4  ?  ?  ? High ?
5 HOCK Clock Input Communication clock from CDC
6 D0~3 Data Both 4-bit command/status data bus
7
8
9
10  ?  ?  ?  ?
11 BCLK? Clock Output Serial bit clock, should be 44100 * 32 = 1.4112MHz
12 SDATA Data Output Serial data
13 LRCK Signal Output I2S word clock, should be 44100Hz. Audio L/R switch for the DAC.
14 D/M ? Signal Output Data/Music. Low during music playback ?
15 GND Power - Ground
16 5V Power - +5V for logic
17 9V Power - +9V for motors and coils
18 GND Power - Ground

Missing: /RESET, C2PO, and 16MHz.

Front-loading model

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

Pin # Name Type Direction Description
1 SGND Power - Ground
2 SVCC Power -
3 VCC Power -
4 WFCK Clock Output Write Frame Clock
5 GND Power - Ground
6 SCOR Pulse Output Subcode sync
7 MUTE  ?
8 SBSO Data Output Subcode data
9 EXCK Clock Input Subcode clock
10 EMP  ?  ? EMPty ?
11 DOUT Data Output  ?
12 6MHz Clock  ?  ?
13 LRCK Clock Output DATA L or R channel (44100Hz)
14 DATA Data  ? Data stream
15 BCLK Clock Output DATA bit clock (44100 * 24 * 2 = 2.12MHz)
16 C2PO Data Output Error flag
17 DB0~3 Data Both 4-bit command/status data bus
18
19
20
21 HOCK Clock Input Communication clock from CDC
22 RESET Pulse Input The name says it all
23 /IRQ Signal Output Asks the CDC for a data exchange
24 L-MUTE  ?
25 CDCK Clock Output Communication clock to CDC

Communication

CDD to CDC communication uses a special protocol handled by the system ROM. Chipicon.png NEO-MGA provides the I/Os through registers Reg.png REG_CDDCTRL, Reg.png REG_CDDINPUT, Reg.png REG_CDDOUTPUT.

Cdunit4bit.png

  • The CDD regularly (~64Hz not 75) 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 set to CDD to CDC.
  • CDCK goes low, indicating that the CDD is starting to send its status data (A).
  • HOCK goes high and the CDC reads the status data (A).
  • CDCK returns high.
  • HOCK returns low.
  • CDCK goes low again, the B word is ready.
  • The cycle continues on for 10 words...

(The delay here is not necessary, it is seen on the NeoGeo CD because of the system ROM preparing stuff).

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

Status data (green)

  • A: Status - 1=playing, 2=seeking, 4=paused, 9=idle, 11=couldn't focus, 14=stopped
  • 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)

  • K: Command
  • L~S: Depends on command (see below)
  • T: 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

See CD drive commands.

Adjustment pots

To do !