Editing Sega CD (Model 1)

{{Infobox computer
| name          = Sega&nbsp;CD&nbsp;Model&nbsp;1&nbsp;("Front-Loader")
| image         = [[File:Sega-Genesis-CD-Model-1-Bare.jpg|260px]]
| caption       = North-American Sega CD Model 1
| manufacturer  = Sega Enterprises Ltd.<br />Optical block by [[Sony]] (KSS-240A/KSS-240R)
| type          = CD-ROM add-on console
| release date = {{Start date|1991|12|12}} (JP&nbsp;Mega-CD)<br />{{Start date|1992|10|15}} (US&nbsp;Sega&nbsp;CD)<br />{{Start date|1993|05|01}} (EU&nbsp;Mega-CD)
| discontinued  = 1994 (superseded by Side-loader & Model 2)
| price         = JP ¥49 800 • US US$299 • EU £270
| cpu           = '''Motorola MC68EC000''' @ 12.500 MHz (primary)<br />'''Sega ASIC “SCD ASIC”''' with embedded 68000 bus arbiter & rotation/scale unit
| coprocessor   = Hitachi HD641709 (“GPP”) 8-bit MCU @ 3.58 MHz (CD-DA sub-CPU & drive control)
| memory        = 768 KB Word-RAM (dual-ported)<br />512 KB Program RAM<br />64 KB Back-up RAM (SRAM, battery)<br />128 KB PCM sample RAM
| storage       = 1× 1 ×-speed CD-ROM (150 kB·s⁻¹)<br />internal back-up RAM (32 KB user-accessible)
| graphics      = Genesis VDP pass-through plus '''CXD1161/CXD1162''' ASICs for rotation, scaling, line-scroll
| 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)
| power         = 9 V-DC 1.2 A (AC adaptor MK-1602 or 1602-1) • < 11 W consumption
| os            = Mega CD System ROM v1.x (128 KB mask ROM)
| compatibility = Plays all Genesis/Mega Drive carts<br />Plays Sega CD/Mega-CD discs (region‐locked)<br />CD-Audio & CD+G graphics discs
| predecessor   = – (first Sega optical system)
| successor     = [[Sega CD (Model 2)]]
| codename      = TMR-50 (Japan R&D), “Topaz” (US)
| model         = HAA-2910 (JP), MK-1601 (US), MK-1690-50 (EU)
}}

The '''Sega CD Model 1''' – marketed in Japan as the '''Mega-CD''' – is the original front-loading compact-disc add-on for the [[Sega Mega_Drive (Model 1)]].  
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.

== 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 ==
{| class="wikitable styled-table" style="width:70%; text-align:center;"
! 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<br />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) ==
{| class="wikitable styled-table" style="width:70%; text-align:center;"
|-
! style="background:#ddd;" |Processing
|-
| Main 68k | 12.500 MHz (25 MHz ÷ 2) wait-stated 3–4 cycles to word-RAM
|-
| Sub MCU | 3.58 MHz (NTSC crystal) – handles CD block decode, seek, audio FIFO
|-
| DMA | 5.0 MB·s⁻¹ peak host⇄word-RAM
|-
! style="background:#ddd;" |Memory map (simplified)
|-
| $0000 00–$1F FFFF | 2 MB Program RAM
|-
| $2000 00–$23 FFFF | 256 KB Word-RAM bank 0
|-
| $2400 00–$27 FFFF | 256 KB Word-RAM bank 1 (dual-port)
|-
| $2FFF E0–$2FFF FF | PCM regs & sample RAM (128 KB windowed)
|-
| $3E00 00–$3E3F FF | Backup SRAM 64 KB (battery)
|-
| $3F00 00–$3F3F FF | ASIC / host control regs
|-
| $3F00 40–$3F00 7F | Sub-CPU I/O FIFO
|-
| $FF00 00–$FFFF FF | 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:

{| class="wikitable styled-table" style="width:70%;"
! Pin !! Dir !! Name !! Description
|-
| B01 ||  → || /AS || Address-strobe
|-
| B02 ||  ← || IRQ2 || CD raises to interrupt Genesis
|-
| B03 || bi || D0 || Data bus (even)
|-
| … || … || … || …
|-
| B25 ||  → || CDCLK || 12.5 MHz clock from Sega CD
|-
| B26 ||  ← || EXCLK || 7.67 MHz clock from Genesis
|-
| B45 ||  ← || AUD-L || PCM mixed left
|-
| B46 ||  ← || AUD-R || PCM mixed right
|-
| B49 ||  – || +5 V ||
|-
| B50 ||  – || GND ||
|}

Full table with all 50 pins on [[Mega Drive expansion port pinout]].

== Known hardware revisions & PCB errata ==
{| class="wikitable styled-table" style="width:90%; text-align:center;"
! 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 || US | BIOS 1.10, word-RAM wait fix || Add R511 pull-up (factory bodge)
|
|-
| 171-5978C || VA2 || EU | 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 CD Service Manual|Sega Service Document #170-0436-01]]

== General maintenance ==
Full disassembly, recap lists and belt change procedure: see [[Sega CD (Model 1) 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.

[[Category:Sega Systems]]