Sega CD (Model 1)
The Sega CD Model 1 – marketed in Japan as the Mega-CD – is the original front-loading compact-disc add-on for the Mega Drive/Genesis. Released in late 1991, it augmented the base console with a faster 12.5 MHz 68EC000, hardware scaling/rotation ASICs, 8-channel PCM audio and the ability to stream full-motion video (Cinepak) from CD-ROM.
| File:SegaCD-Model1-Unit.jpg | |
| North-American Sega CD Model 1 attached to an early VA2 Genesis Model 1 | |
| Specifications | |
|---|---|
| Manufacturer | Sega Enterprises Ltd. Optical block by Sony (KSS-240A/KSS-240R) |
| Type | CD-ROM add-on console |
| Released | December 12, 1991 (JP Mega-CD) October 15, 1992 (US Sega CD) |
| Discontinued | 1994 (superseded by Side-loader & Model 2) |
| Intro price | JP ¥49 800 • US US$299 • EU £270 |
| CPU | Motorola MC68EC000 @ 12.500 MHz (primary) Sega ASIC “SCD ASIC” with embedded 68000 bus arbiter & rotation/scale unit |
| Memory | 768 KB Word-RAM (dual-ported) 512 KB Program RAM 64 KB Back-up RAM (SRAM, battery) 128 KB PCM sample RAM |
| Storage | 1× 1 ×-speed CD-ROM (150 kB·s⁻¹) internal back-up RAM (32 KB user-accessible) |
| Display | 320 × 224 / 256 × 224 px, 5-bit RGB, 128 sprites (Genesis VDP) |
| Sound | Genesis PSG & YM2612 FM plus Ricoh RF5C164 8-ch PCM @ 32 kHz • 10-bit DAC |
| Dimensions | 294 mm W × 212 mm D × 80 mm H |
| Weight | ≈ 2.0 kg (drive + PSU shielding) |
| OS / Firmware | Mega CD System ROM v1.x (128 KB mask ROM) |
| Predecessor | – (first Sega optical system) |
| Successor | Sega CD (Model 2) / Wondermega / CDX |
| Codename | TMR-50 (Japan R&D), “Topaz” (US) |
| Model no. | HAA-2910 (JP), MK-1601 (US), MK-1690-50 (EU) |
🕰️ Historical context & sales
- Concept (1989–1990): Sega R&D2 (Hisashi Suzuki) partnered with Sony Devices Div. to adopt the KSS-240 laser deck.
- Launch: 12 Dec 1991, JP ¥49 800 – initial shipment 100 k; supply shortages until Feb 1992.
- US debut: 15 Oct 1992 at US$299 bundled with ‘’Sewer Shark’’ (first US CD-ROM pack-in).
- Total Model 1 units: ≈ 600 k worldwide (of 2.2 M total Sega CD family).
🧠 Architecture overview
| Sub-system | IC(s) | Notes |
|---|---|---|
| Primary CPU | MC68EC000FN12 | 16-bit, no MMU, bus-master shares Genesis address lines |
| Scaling / Rotation | Sony CXD1161P + CXD1162P | 1024×1024 pixel source; affine transform to 320×224 |
| PCM audio | Ricoh RF5C164 (QFP-80) | 8 voices, 10-bit DAC, 32 kHz max |
| CD controller | Hitachi HD641709 MCU (a 6301) + Sega CXD1800 drive DSP | Sub-CPU executes CD-DA and ISO9660 BIOS |
| Gate-array | Sega 315-5548 (ASIC) | Bus control, battery SRAM decode, host port, DMA to word-RAM |
| RAM | 2 × OKI M514256 (256 K×4) word-RAM 8 × HM514170 (64 K×4) program-RAM |
Dual-port by ASIC |
| Optical deck | Sony PU-18 mech. w/ KSS-240A lens | 1 × CLV, spindle @ 530 rpm (innermost) |
📊 Technical specification (Model 1, BIOS v1.10)
| Processing | |
|---|---|
| 12.500 MHz (25 MHz ÷ 2) wait-stated 3–4 cycles to word-RAM | |
| 3.58 MHz (NTSC crystal) – handles CD block decode, seek, audio FIFO | |
| 5.0 MB·s⁻¹ peak host⇄word-RAM | |
| Memory map (simplified) | |
| 2 MB Program RAM | |
| 256 KB Word-RAM bank 0 | |
| 256 KB Word-RAM bank 1 (dual-port) | |
| PCM regs & sample RAM (128 KB windowed) | |
| Backup SRAM 64 KB (battery) | |
| ASIC / host control regs | |
| Sub-CPU I/O FIFO | |
| BIOS ROM 128 KB (mirrored) | |
Video timing
- Pixel clock 13.5 MHz (NTSC) / 13.32 MHz (PAL) via VDP master.
- Scaled plane generated at 256×A & line-buffered into Genesis VDP CRAM.
- V-blank interrupt every 262 lines NTSC (≈ 59.92 Hz).
🔌 Pin-by-pin expansion bus
The 50-pin edge (CN-B) carries the Genesis 68000 bus plus additional DMA and audio lines:
| Pin | Dir | Name | Description | ||||||
|---|---|---|---|---|---|---|---|---|---|
| → | /AS | Address-strobe | ← | IRQ2 | CD raises to interrupt Genesis | bi | D0 | Data bus (even) | … | … | … | → | CDCLK | 12.5 MHz clock from Sega CD | ← | EXCLK | 7.67 MHz clock from Genesis | ← | AUD-L | PCM mixed left | ← | AUD-R | PCM mixed right | – | +5 V | | – | GND | |
Full table with all 50 pins on Mega Drive expansion port pinout.
🛠️ Known hardware revisions & PCB errata
| Assy # | Board silk | Region | Notable changes / bugs | Service fixes |
|---|---|---|---|---|
| 171-5978A | VA0 | JP launch | KSS-240A lens, BIOS 1.00 | CD door switch mis-adjust – bend lever |
| 171-5978B | VA1 | BIOS 1.10, word-RAM wait fix | Add R511 pull-up (factory bodge) | |
| 171-5978C | VA2 | CXD1162 rev-C, 315-5548A ASIC | 74HC00 (IC15) ESD prone – replace with HCT |
🔧 Common failure points
- “GREEN LED but no spin” – blown surface-mount fuse F301 (2 A) or shorted 5 V tantalum C230.
- Tray won’t eject / belt slip – 40 mm × 1.2 mm neoprene belt hardened. Replace with 41 mm 0.6 mm square belt.
- No CD audio – CXD1162P pin 47 mute line held low; open Q203 (mute transistor) or BIOS stuck (bad RAM).
- Distorted PCM – leaking SMD electrolytics around RF5C164: C510 /C511 47 µF.
- “ERROR 02” (seek timeout) – dirty rails, laser current low (VR102 ≈ 50 mV across TP100/101).
- Backup RAM corrupt – CR2032 coin cell dead or IC3 SRM2016 failing.
🔩 Service manuals & schematics
- Sega Service Document #170-0436-01 (download mirrored at SegaRetro).
- Sony PU-18 optical deck manual (exploded view, focus gain pots).
- Annotated board scans on github.com/arkagame/mega-cd-archivist.
🧰 General maintenance
Full disassembly, recap lists and belt change procedure: see Sega CD (Model 1) General Maintenance Guide. Capacitor map and polarity silks are on Sega CD (Model 1) Capacitor Replacement Guide.
🩺 Troubleshooting
A step-by-step power-on diagnostic flow (LED blink codes, fuse chart, clock probing) is available on Sega CD (Model 1) Troubleshooting Guide.
🔉 Audio/Video modifications
- Direct-mix “board-jail”: lift R171 &R172, route RF5C164 output to Genesis EXT stereo jack – no crosstalk.
- RGB bypass: use Genesis VDP RGB; Sega CD adds no extra graphics plane, so standard MD RGB mod applies.
- 50/60 Hz region switch – tie ASIC pin 4 ‘AREA’ to DIP switch + patch BIOS byte $1FE1.
📀 Software & homebrew
Model 1 BIOS 1.10 supports CD-R if session is MODE1+CDDA (avoid 40× burns). Recommended test suites: ‘’Sega CD Test Disc’’ by discedge; ‘’RAM Stress’’ by Luke Usher.
🎲 Trivia & pop-culture
- ‘’Snatcher’’ (JP) used SCD PCM channels for full Japanese voice acting – a first on 8 Mbit disc.
- The launch bundle ‘’Sol-Feace’’ shipped on both cart (JP) and CD; players compared load-times as marketing.
- In TV series ‘’Clarissa Explains It All’’ (1993) a front-loader Sega CD is visible under the CRT.