Editing Sega CD (Model 2)

{{Infobox computer
| name          = Sega&nbsp;CD&nbsp;Model&nbsp;2<br />(Mega-CD&nbsp;2)
| image         = [[File:Sega-CD-Model2-Set.png|260px]]
| caption       = U.S. NTSC Sega CD (Model 2) side-mounted beneath a Genesis 2
| manufacturer  = Sega Enterprises Ltd.
| type          = CD-ROM add-on console
| release date  = {{Start date|1993|04|23}} (JP Mega-CD 2)<br />{{Start date|1993|11}} (US Sega CD 2)<br />{{Start date|1994|05}} (EU Mega-CD 2)
| discontinued  = 1996 (replaced by licensed JVC WonderMega 2 & CD X)
| price         = JP ¥29 800 • US US$229 • EU £230
| cpu           = '''Motorola MC68EC000''' @ 12.500 MHz (main)<br />'''Sony CXD1802Q''' custom gate-array (CD-DSP) †
| coprocessor   = '''Hitachi HD641709-PS6 (“GPP”)''' 8-bit MCU – drive & CD-DA control
| memory        = 512 KB Program RAM<br />768 KB dual-ported Word-RAM<br />128 KB PCM Sample RAM<br />64 KB Battery-backed Save RAM
| storage       = 1× 1 ×-speed top-loading CD-ROM (150 kB s⁻¹)<br />cartridge passthrough up to 4 MB
| graphics      = Genesis VDP pass-through + '''315-5880''' ASIC (rotation, scaling, B-B blend)
| 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
| power         = 9 V-DC 1.2 A (AC Adaptor MK-1602 / MK-2103) • ‹ 10 W
| os            = Mega-CD System BIOS v2.x (128 KB mask ROM)
| compatibility = Plays Mega CD/Sega CD discs (region-locked) • CD-DA • CD+G<br />Pass-through for all Genesis/Mega Drive carts • Compatible with 32X add-on
| predecessor   = [[Sega CD (Model 1)|Sega CD (Model 1)]]
| successor     = [[Sega CDX]] / [[JVC Wondermega]]
| codename      = • ''MARS-CD-B'' (JP R&D) • “Top-loader”
| model         = 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 ==
{| class="wikitable" style="width:70%; text-align:center;"
|+'''Major ICs (VA0 main-board)'''
! 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) ===
{| class="wikitable" style="width:70%;"
|+'''Sega CD Model 2 Physical Address Space'''
! 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 ==
{| class="wikitable" style="width:70%;"
! 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 ==
{| class="wikitable" style="width:90%; text-align:center;"
|+'''Mega-CD 2 Main-Boards'''
! 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 ==
{| class="wikitable" style="width:96%;"
! 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 ==
* [[Sega CD 2 / Mega CD 2 - Service Manual]] – 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)|Sega CD Model 1]] – original front-loader  
* [[Sega Mega Drive / Genesis Model 2|Sega Genesis (Model 2)]] – matching base console  
* [[Sega 32X]] – 32-bit attachment (Model 2 compatible)

[[Category:Sega Systems]]