Sega CD (Model 2): Difference between revisions
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== 🧠 Hardware Architecture == | == 🧠 Hardware Architecture == | ||
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! Part !! Function(s) | ! Part !! Function(s) | ||
Revision as of 21:42, 27 April 2025
| File:Sega-CD-Model-2-Top.jpg | |
| U.S. NTSC Sega CD (Model 2) side-mounted beneath a Genesis 2 | |
| Specifications | |
|---|---|
| Manufacturer | Sega Enterprises Ltd. |
| Type | CD-ROM add-on console |
| Released | April 23, 1993 (JP Mega-CD 2) November 1, 1993 (US Sega CD 2) |
| Discontinued | 1996 (replaced by licensed JVC WonderMega 2 & CD X) |
| Intro price | JP ¥29 800 • US US$229 • EU £230 |
| CPU | Motorola MC68EC000 @ 12.500 MHz (main) Sony CXD1802Q custom gate-array (CD-DSP) † |
| Memory | 512 KB Program RAM 768 KB dual-ported Word-RAM 128 KB PCM Sample RAM 64 KB Battery-backed Save RAM |
| Storage | 1× 1 ×-speed top-loading CD-ROM (150 kB s⁻¹) cartridge passthrough up to 4 MB |
| Display | 256 / 320 × 224 px (NTSC) • 52-colour line-scroll • 80 sprites |
| Sound | Genesis YM3438 FM & PSG plus Ricoh RF5C164 8-ch PCM @ 32 kHz 10-bit DAC |
| Dimensions | 252 mm W × 215 mm D × 63 mm H |
| Weight | ≈ 1.3 kg |
| OS / Firmware | Mega-CD System BIOS v2.x (128 KB mask ROM) |
| Predecessor | Sega CD (Model 1) |
| Successor | Sega CDX / JVC Wondermega |
| Codename | • MARS-CD-B (JP R&D) • “Top-loader” |
| Model no. | HAA-2950 (JP) • MK-4102 (US) • MK-4102-50 (EU) |
The Sega CD Model 2 – known in Japan and PAL regions as the Mega-CD 2 – is the second-generation optical add-on for the Sega Genesis/Mega Drive. Launched **April 1993** in Japan and late-1993 overseas, it re-engineers the bulky front-loading Model 1 into a **smaller, top-loading deck** that bolts to the right-hand side of a Model-2 Genesis (or underneath a Model-1 via spacer plate). The redesign slashes build cost by integrating disc-logic, FM synth mix, and RF modulator into two new ASICs and by replacing the complex motorised tray with a spring-latched lid.
📜 Historical Context & Sales
- Sega’s first CD drive – the **Model 1 “front-loader”** (HAA-2910) – cost US$299 and carried a high RMA rate (optical alignment, belt stretch).
- Cost-reduction taskforce (“MCD-R2”) produced a simplified mechanism using Sony’s ubiquitous **KSS-210A** pickup and a plastic clam-shell housing.
- Debuted 23 April 1993 bundled with Sonic CD in Japan; U.S. release November 1993 bundled with Sewer Shark.
- Combined Model-1/2 worldwide sales ≈ **2.24 million** units – ~70 % are Model 2 according to 1995 Sega annual report.
🧠 Hardware Architecture
| Part | Function(s) |
|---|---|
| MC68EC000 (64-pin) | Primary 16-bit CPU @ 12.5 MHz |
| 315-5880 “MCD ASIC 2” | Sub-CPU bus arbiter • Word-RAM controller • rotation / scaling / 3×3 matrix |
| 315-5788 | OKI MSM6295-compatible DMA + CD-SUB code ROM |
| Ricoh RF5C164 | 8-channel PCM generator, 10-bit DAC |
| CXD2510Q | CD-DSP / decoder – replaces discrete CN-303 & CXP controllers in Model 1 |
| 315-5684-B | RGB DAC + stereo mixer (shares with Genesis 2) |
Internally, software sees **identical registers** to Model 1; all games and dev tools remain compatible.
Clock Scheme (NTSC)
- Master XTAL = **12.500 MHz × 4 = 50.000 MHz** → ÷4 = 12.5 MHz (68k)
- Z80 (in Genesis) still sourced from 53.693 MHz ÷15 ≈ 3.58 MHz
- Word-RAM dual-ported clock = 25 MHz.
Memory Map (MC68EC000)
| Range | Size | Device |
|---|---|---|
| $0000 00–$0FFF FF | 1 MB | Program RAM |
| $0200 00–$03FF FF | 2 MB | Word-RAM (dual-bank 0/1) |
| $0400 00–$05FF FF | 2 MB | PRG-RAM mirror / save RAM (battery) |
| $0600 00–$07FF FF | 2 MB | PCM sample RAM mirror |
| $0800 00–$0BFF FF | 4 MB | BIOS ROM (128 KB mirrored) |
| $0C00 00–$0DFF FF | 2 MB | CD-DSP & sub-CPU registers |
| $0E00 00–$0E00 FF | 256 B | Gate array I/O (gate, LEDs, reset) |
🔄 Differences vs. Model 1
| Area | Model 1 (1991) | Model 2 (1993) | Result |
|---|---|---|---|
| Drive mech | Sony KSS-240A sled + worm gear front-tray | Sony KSS-210A fixed sled, top-lid | 40 % cheaper, quieter |
| Optical control | CXP921P + CXD1095Q (DSP) | **CXD2510Q** SoC | Fewer PCB layers |
| Audio mixing | External op-amps, RCA output | Mixed inside **315-5684-B**, no RCA | Shares Genesis 2 mini-DIN |
| Power rails | 9 V in → 5 V, –5 V, +12 V regulators | –5 V rail deleted (unused) | One less 7905 IC |
| Word-RAM width | 2 × 16-bit SRAM (dual bus) | Same, but SRAMs integrated in ASIC later VA1 | Slightly lower latency |
🔌 External Connectors
- **Expansion edge** – identical 64-pin BUS as Model 1; ships with **10 cm ribbon “mixing” cable** to the Genesis 2 A/V port (carries left/right audio).
- **AC Adaptor** – same barrel 2.1 mm as Genesis 2 (centre positive).
- **EXT Link** – 9-pin mini-DIN to daisy-chain two Mega-CDs for dev use (never utilised by retail titles).
📐 Motherboard Revisions
| Rev | Key ICs | Notes |
|---|---|---|
| VA0 | 315-5880, CXD2510Q, discrete SDRAM | Japan launch – green soldermask |
| VA1 | **315-5933 “ASIC 3”** (RAM+gate array in one) | Most common US/EU; no Word-RAM wait glitches |
| VA2 (Majesco) | GOAC single-chip + Mitsubishi drive board | Pack-in with late Genesis 3; cost-reduced |
- All revisions are cross-compatible; BIOS ROM changes (v2.00→v2.11) fix **Lunar Eternal Blue** save bug and 32X pass-through timing.*
🔊 Audio Path
1. RF5C164 PCM → 315-5880 bus → 2. ASIC mixer combines FM+PSG from Genesis via ribbon audio BUS → 3. 315-5684-B internal op-amp → mini-DIN 9 (pins 2/4) / RCA L+R **(PAL only)**. Because mixing is line-level only, Model 2 inherits Genesis 2’s famous “quiet” output (~300 mVpp). Mods: VMU Audio Amp or “Sega CD Audio Boost” board (TL072).
🛠️ Maintenance
- **Pickup lens** – KSS-210A rarely fails; clean with 91 % IPA swab.
- **Spindle hub** – plastic hub cracks; 3-D-print “clamp hub saver” or cannibalise PC CD-ROM.
- **Lid switch** – dual leaf SW1; adjust if unit reports “Please Close Cover” error.
- **Belt** – Model 2 has *no belt*, only a direct-drive spindle.
- **Capacitors** – far fewer SMD cans: primarily C224 220 µF (5 V rail) and audio coupling 47 µF (C138, C139). See Sega CD Model 2 Capacitor Replacement Guide.
🐞 Common Faults & Fixes
| Symptom | Diagnosis | Remedy |
|---|---|---|
| “Checking disc” endless | Spindle hub height / dirty lens | Clean lens; shim hub 0.2 mm |
| No PCM channels | RF5C164 pin-rot on VA0 | Re-flow QFP; recap C139/C140 |
| BIOS boots, drive dead | 5 V present, 9 V missing at CN3 | Replace poly-fuse PF1 or AC adaptor |
| 32X hangs on Sega CD | ASIC revision < v2.11 | Burn patch cart with “32X-SCD fix” |
🔧 Popular Mods
- **RGB Bypass** via Triple-Bypass board (VDP → THS7374 → DIN 9).
- **Region Free BIOS** – swap 27C1001 EPROM with patched Mega CD Multi-BIOS v2.8.
- **Non-destructive Switchless** – PIC16F630 toggles JP1/JP2 for 50/60 Hz & J/U/E.
- **S-Video + Stereo Jack** – tap Y/C from CXD2510Q and RF5C164 out.
📚 Technical Documentation
- *Mega-CD 2 Service Manual* (Sega doc 400-6408-01) – full schematics & test points.
- Yasushi Yamaguchi, “Sega CD ASIC Architecture” – Nikkei 1994 conference slides.
- US Patent 5 471 206 – “Home-use CDROM system with rotation & scale processing”.
🗞️ Trivia & Pop-Culture
- Sega’s marketing touted Model 2 as “**2 ½ times faster**” to load due to firmware tweaks, though the drive itself is identical 1 × speed.
- Film prop in Hackers (1995) – Acid Burn’s bedroom shows a Mega-CD 2 + Genesis 2 stack.
- The JP manual warns: “Do not place cup noodles on top lid while operating.”
🔗 See Also
- Sega CD Model 1 – original front-loader
- Sega Genesis (Model 2) – matching base console
- Sega 32X – 32-bit attachment (Model 2 compatible)