Sega Mega Drive / Genesis (Model 1): Difference between revisions
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{{Infobox computer | {{Infobox computer | ||
| name = Sega Mega Drive – Model 1 | | name = Sega Mega Drive – Model 1 | ||
| image = [[File:Sega-Mega-Drive- | | image = [[File:Sega-Mega-Drive-JP-Mk1-Console-Set.jpg|250px]] | ||
| caption = First-generation (“High Definition Graphics – Stereo Sound”) Mega Drive released in Japan & Europe | | caption = First-generation (“High Definition Graphics – Stereo Sound”) Mega Drive released in Japan & Europe | ||
| manufacturer = Sega Enterprises, Ltd. | | manufacturer = Sega Enterprises, Ltd. | ||
| Line 10: | Line 10: | ||
| cpu = Motorola 68000 @ 7.67 MHz<ref>NTSC master clock 53.693175 MHz ÷7.</ref><br />Zilog Z80 @ 3.58 MHz (audio & Master-System compatibility) | | cpu = Motorola 68000 @ 7.67 MHz<ref>NTSC master clock 53.693175 MHz ÷7.</ref><br />Zilog Z80 @ 3.58 MHz (audio & Master-System compatibility) | ||
| memory = 64 KB 16-bit Work RAM • 64 KB VRAM • 8 KB Z80 RAM | | 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)<br />CD-ROM via [[Sega | | storage = ROM cartridges (up to 4 MB officially; larger with bank-switch)<br />CD-ROM via [[Sega CD (Model 1)]] add-on | ||
| display = 320 × 224 px (256 × 224, 320 × 240 & 256 × 240 modes) • 64 colours onscreen from 512-colour palette • 80 sprites | | 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) | | sound = Yamaha '''YM2612''' 6-ch FM + SN76489 PSG 4-ch + 8-bit PCM (via 68000 DMA) | ||
| Line 16: | Line 16: | ||
| weight = 1.2 kg | | weight = 1.2 kg | ||
| os = None (game cartridge provides code) | | os = None (game cartridge provides code) | ||
| predecessor = [[ | | predecessor = [[Master System II]] | ||
| successor = [[Sega | | successor = [[Sega Mega Drive / Genesis (Model 2)]] | ||
| codename = '''MK-1601''' (HAA-2510 PCB) | | codename = '''MK-1601''' (HAA-2510 PCB) | ||
| model = “VA0–VA6” motherboard revisions | | model = “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. | 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 == | ||
{| class="wikitable styled-table" style="width:70%; text-align:center;" | {| class="wikitable styled-table" style="width:70%; text-align:center;" | ||
! Sub-system !! Specification (Model 1 VA4, NTSC) | ! Sub-system !! Specification (Model 1 VA4, NTSC) | ||
| Line 47: | Line 46: | ||
|} | |} | ||
== | == Memory Map (NTSC, no add-ons) == | ||
{| class="wikitable styled-table" style="width:70%;" | {| class="wikitable styled-table" style="width:70%;" | ||
|+'''Mega Drive Model 1 Address Space (68000)''' | |+'''Mega Drive Model 1 Address Space (68000)''' | ||
| Line 75: | Line 74: | ||
Key ROM vectors: <code>000004</code> initial SSP, <code>000006</code> initial PC (game entry), TMSS “SEGA” string check at <code>000100</code> on later VA6(+). | Key ROM vectors: <code>000004</code> initial SSP, <code>000006</code> initial PC (game entry), TMSS “SEGA” string check at <code>000100</code> on later VA6(+). | ||
== | == Video Timing == | ||
{| class="wikitable styled-table" style="width:70%; text-align:center;" | {| class="wikitable styled-table" style="width:70%; text-align:center;" | ||
! Mode !! Pixel clock !! Pixels/line !! Lines/frame !! Refresh | ! Mode !! Pixel clock !! Pixels/line !! Lines/frame !! Refresh | ||
| Line 84: | Line 83: | ||
|} | |} | ||
''The VDP fetches one 16-bit VRAM word every 4 cycles; HSYNC width ≈ 84 master clocks.'' | |||
== | == Edge Connector (64-pin Cartridge Slot) == | ||
{| class="wikitable styled-table" style="width:90%; text-align:center;" | {| class="wikitable styled-table" style="width:90%; text-align:center;" | ||
! Pin !! Signal !! Description !! Pin !! Signal !! Description | ! Pin !! Signal !! Description !! Pin !! Signal !! Description | ||
| Line 103: | Line 102: | ||
|} | |} | ||
''Full 64-pin table located on the [[Mega Drive Cartridge Pin-out]] page.'' | |||
== | == Model 1 Motherboard Revisions == | ||
{| class="wikitable styled-table" style="width:80%; text-align:center;" | {| class="wikitable styled-table" style="width:80%; text-align:center;" | ||
|+'''Key Mega Drive VA-board Changes''' | |+'''Key Mega Drive VA-board Changes''' | ||
| Line 119: | Line 118: | ||
|} | |} | ||
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 [[Sega Mega Drive Maintenance Guide|maintenance guide]] for board-specific recap lists. | 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 [[Sega Mega Drive / Genesis (Model 1) Maintenance Guide|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. | 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]]). | *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. | * 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. | * Japanese uptake was modest (≈ 400 k units by 1990) until ''Sonic the Hedgehog'' (1991) exploded sales. | ||
| Line 131: | Line 130: | ||
* Sixth-year production cost with VA6 board < US$50, leading to sub-£100 “Mega Drive II” bundles in 1995. | * 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]]. | * '''Electrolytic capacitor leakage''' around CXA1034 (audio) and power rails (5 V, -12 V op-amp). See [[Sega Mega Drive / Genesis (Model 1) Capacitor Replacement Guide]]. | ||
* '''No video / jailbars''' – cracked solder on AV DIN, VDP or RAM ICs. | * '''No video / jailbars''' – cracked solder on AV DIN, VDP or RAM ICs. | ||
* '''RESET line stuck low''' – faulty MC14069 (Hex Schmitt inverter) on VA0-VA3. | * '''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. | * '''Z80 bus contention''' causes garbled FM – reseat / clean edge cartridge & check 315-5410 ASIC. | ||
* Full troubleshooting flow-chart on [[Mega Drive Troubleshooting Guide]]. | * Full troubleshooting flow-chart on [[Sega Mega Drive / Genesis (Model 1) Troubleshooting Guide]]. | ||
== | == Expansion & I/O == | ||
* '''Cartridge slot''' 64-pin edge (mapped @ 0–4 MB) | * '''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) | * '''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) | * '''AV Out''' 8-pin mini-DIN (RGB 0.7 Vpp, composite, mono audio) | ||
* '''Controller ports''' 2× DE-9 (compatible with Atari/VCS & MS pads) | * '''Controller ports''' 2× DE-9 (compatible with Atari/VCS & MS pads) | ||
* '''Add-ons:''' [[Sega | * '''Add-ons:''' [[Sega CD (Model 1)]] (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. | * 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. | * 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 <code>A+B+C+START</code> on power-up with no cartridge inserted (development boards only). | * An undocumented VDP test pattern can be triggered by holding <code>A+B+C+START</code> on power-up with no cartridge inserted (development boards only). | ||
== Further reading / guides == | |||
* [[Sega Mega Drive / Genesis (Model 1) Maintenance Guide]] – disassembly, PSU voltages, cleaning | |||
* [[Sega Mega Drive / Genesis (Model 1) Troubleshooting Guide]] – no-power, black-screen, audio issues | |||
* [[Sega Mega Drive Maintenance Guide]] – disassembly, PSU voltages, cleaning | * [[Sega Mega Drive / Genesis (Model 1) Capacitor Replacement Guide]] – board-specific recap BOM | ||
* [[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 | * [[Sega Mega Drive AV Output Guide]] – RGB, composite, stereo mod & jailbar fixes | ||
<references /> | <references /> | ||
[[Category:Sega Systems]] | [[Category:Sega Systems]] | ||
Latest revision as of 09:52, 12 May 2025
 | |
| 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[1] 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 CD (Model 1) 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 | Master System II |
| Successor | Sega Mega Drive / Genesis (Model 2) |
| 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
[edit | edit source]| 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)
[edit | edit source]| 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
[edit | edit source]| 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)
[edit | edit source]| 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
[edit | edit source]| 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
[edit | edit source]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
[edit | edit source]- 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
[edit | edit source]- Electrolytic capacitor leakage around CXA1034 (audio) and power rails (5 V, -12 V op-amp). See Sega Mega Drive / Genesis (Model 1) 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 Sega Mega Drive / Genesis (Model 1) Troubleshooting Guide.
Expansion & I/O
[edit | edit source]- 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 CD (Model 1) (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
[edit | edit source]- 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+STARTon power-up with no cartridge inserted (development boards only).
Further reading / guides
[edit | edit source]- Sega Mega Drive / Genesis (Model 1) Maintenance Guide – disassembly, PSU voltages, cleaning
- Sega Mega Drive / Genesis (Model 1) Troubleshooting Guide – no-power, black-screen, audio issues
- Sega Mega Drive / Genesis (Model 1) Capacitor Replacement Guide – board-specific recap BOM
- Sega Mega Drive AV Output Guide – RGB, composite, stereo mod & jailbar fixes
- ↑ NTSC master clock 53.693175 MHz ÷7.