Sega Mega Drive / Genesis (Model 1)

Sega Mega Drive – Model 1

First-generation (“High Definition Graphics – Stereo Sound”) Mega Drive released in Japan & Europe
Specifications
Manufacturer	Sega Enterprises, Ltd.
Type	16-bit home video-game console
Released	29 October 1988 (JP) 14 August 1989 (NA – as Genesis) 30 November 1990 (EU)
Discontinued	April 1997 (worldwide)
Intro price	JP ¥21 000 • US US$189 (1989) • UK £189.99 (1990)
CPU	Motorola 68000 @ 7.67 MHz Zilog Z80 @ 3.58 MHz (audio & Master-System compatibility)
Memory	64 KB 16-bit Work RAM • 64 KB VRAM • 8 KB Z80 RAM
Storage	ROM cartridges (up to 4 MB officially; larger with bank-switch) CD-ROM via Sega Mega-CD add-on
Display	320 × 224 px (256 × 224, 320 × 240 & 256 × 240 modes) • 64 colours onscreen from 512-colour palette • 80 sprites
Sound	Yamaha YM2612 6-ch FM + SN76489 PSG 4-ch + 8-bit PCM (via 68000 DMA)
Dimensions	280 mm W × 210 mm D × 70 mm H
Weight	1.2 kg
OS / Firmware	None (game cartridge provides code)
Predecessor	Sega Master System
Successor	Sega Saturn
Codename	MK-1601 (HAA-2510 PCB)
Model no.	“VA0–VA6” motherboard revisions

The Sega Mega Drive—released as the Sega Genesis in North America—is a 16-bit fourth-generation console that propelled Sega into direct competition with Nintendo’s Super Famicom/SNES. Its Motorola 68000 CPU and sophisticated Yamaha FM audio gave arcade-like performance at home, while aggressive “Genesis does what Nintendon’t” marketing helped drive ≈ 30 million units sold worldwide.

🧠 Architecture & Dual-CPU Design

Sub-system	Specification (Model 1 VA4, NTSC)
Main CPU	Motorola 68000 @ 7.67 MHz (16/24-bit CISC, 68010-class core)
Sound CPU / 8-bit legacy	Zilog Z80 @ 3.579 MHz – controls PSG & streams PCM to YM2612 DAC Also hosts complete Master-System I/O for backward compatibility (cartridge mode pin M3)
VDP	Sega Custom YM7101 (315-5313) – derive from Yamaha V9938: 2 scrolling BG planes + window + 80 sprites, 64 colours onscreen
Audio	Yamaha YM2612 (OPN2) 6-ch FM + integrated 8-bit DAC (channel 6) Texas Instruments SN76489 PSG (3 tone + noise) clocked from 3.58 MHz ÷ 16
RAM	2 × 32 KB 16-bit DRAM (Work RAM) 2 × 32 KB VRAM (dual-ported) 8 KB static RAM for Z80
ROM space	Up to 4 MB linear; larger games (Virtua Racing, 6 MB) use mapper ASIC
DMA	68000→VDP HSYNC DMA (13.4 MB/s burst) and VRAM copy modes

💾 Memory Map (NTSC, no add-ons)

Mega Drive Model 1 Address Space (68000)

Range	Size	Purpose
000000–3FFFFF	4 MB	Cartridge ROM / SRAM
400000–7FFFFF	Mirrors / future mapper
A00000–A0FFFF	64 KB	Z80 address space (via bus request)
A10000–A1001F	32 B	I/O ports: controller, version, TMSS
A11100	1 B	Bus-grant register (68k⇄Z80)
A11200	1 B	Z80 reset
C00000–C0001F	32 B	VDP registers
C00000–C0FFFF	64 KB	VRAM (through VDP)
C00020–C0003F	32 B	PSG, YM2612, CRTC HV counter
FF0000–FFFFFF	64 KB	System RAM (work RAM)

Key ROM vectors: 000004 initial SSP, 000006 initial PC (game entry), TMSS “SEGA” string check at 000100 on later VA6(+).

📺 Video Timing

Mode	Pixel clock	Pixels/line	Lines/frame	Refresh
NTSC 320-wide	10.738 MHz	342 active / 427 total	262	59.922 Hz
PAL 320-wide	10.054 MHz	342 / 427	312	49.701 Hz

• The VDP fetches one 16-bit VRAM word every 4 cycles; HSYNC width ≈ 84 master clocks.*

🔌 Edge Connector (64-pin Cartridge Slot)

Pin	Signal	Description	Pin	Signal	Description
A1	GND	Ground	B1	+5 V
A2	A23	Address bus 23	B2	A22
A3	A21	…	B3	A20
⋯	⋯	⋯	⋯	⋯	⋯
A30	M3	Low = Master-System mode	B30	/CART_INH	Pull low to disable ROM (CD/32X)
A32	/OE	16-bit ROM output enable	B32	/WE (SRAM)

• Full 64-pin table located on the Mega Drive Cartridge Pin-out page.*

🛠️ Model 1 Motherboard Revisions

Key Mega Drive VA-board Changes

Rev (JP)	ASIC	Notable characteristics
VA0 (launch, 1988)	315-5211 VDP	Rare; discrete RGB amp, noisier audio
VA2–VA4 (1989)	315-5313 VDP	Adds low-pass in audio, fixes VDP register 1 bug
VA5 (1991)	315-5476 combined I/O	˜1 dB louder YM2612 DAC, TMSS “LOCK-ON”
VA6 (1992)	same	TMSS mandatory; “Licensed by Sega” BIOS check

VA4 & later drop the “EXT PORT” text on the front but keep the 9-pin SUB-D connector. Revision affects RGB quality and capacitor layout—see the dedicated maintenance guide for board-specific recap lists.

🔉 Audio Path

68000 DMA → YM2612 FM → mixed with PSG → pre-emphasis RC filter → Sony CXA1034 op-amp → headphone amp & rear AV-out.

• Early VA0/VA1 units suffer the well-known “Mega-Drive whistle.”* Installing 470 µF / 100 µF low-ESR caps and moving the audio return after the regulators mitigates it (documented in the Mega Drive AV Output Guide).

🕰️ Historical Context & Sales

• Conceived under codename “Mark V” to supersede the 8-bit Master System using Sega’s System-16 arcade silicon.
• Japanese uptake was modest (≈ 400 k units by 1990) until Sonic the Hedgehog (1991) exploded sales.
• North America (Genesis): > 18 million; Europe/Middle East: > 8 million; Brazil (Tectoy): 3 million+.
• Sixth-year production cost with VA6 board < US$50, leading to sub-£100 “Mega Drive II” bundles in 1995.

⚠️ Common Faults & Preventive Maintenance

• Electrolytic capacitor leakage around CXA1034 (audio) and power rails (5 V, -12 V op-amp). See Mega Drive Capacitor Replacement Guide.
• No video / jailbars – cracked solder on AV DIN, VDP or RAM ICs.
• RESET line stuck low – faulty MC14069 (Hex Schmitt inverter) on VA0-VA3.
• Z80 bus contention causes garbled FM – reseat / clean edge cartridge & check 315-5410 ASIC.
• Full troubleshooting flow-chart on Mega Drive Troubleshooting Guide.

🎮 Expansion & I/O

• Cartridge slot 64-pin edge (mapped @ 0–4 MB)
• EXT PORT 9-pin mini-DIN (unused by most games; utilised by Sega CD side-bus)
• AV Out 8-pin mini-DIN (RGB 0.7 Vpp, composite, mono audio)
• Controller ports 2× DE-9 (compatible with Atari/VCS & MS pads)
• Add-ons: Sega Mega-CD (CD-ROM storage, additional 12.5 MHz 68000 & PCM) and Sega 32X (2× Hitachi SH-2 @ 23 MHz, 32-colour cell-based renderer)

🎬 Trivia & Pop-culture

• Early Japanese units proudly display “High Definition Graphics • Stereo Sound” silk-screen; later cost-reduced versions removed the legend though hardware remained identical.
• The console appears (model-accurate) in the 2020 film Sonic the Hedgehog—complete with a VA3 style headphone volume wheel.
• An undocumented VDP test pattern can be triggered by holding A+B+C+START on power-up with no cartridge inserted (development boards only).

---

Further reading / guides

• Sega Mega Drive Maintenance Guide – disassembly, PSU voltages, cleaning
• Sega Mega Drive Troubleshooting Guide – no-power, black-screen, audio issues
• Sega Mega Drive Capacitor Replacement Guide – board-specific recap BOM
• Sega Mega Drive AV Output Guide – RGB, composite, stereo mod & jailbar fixes