Sega Master System II
 | |
| European PAL “SMS II” (Model 3000-07) with integrated Alex Kidd in Miracle World' | |
| Specifications | |
|---|---|
| Manufacturer | Sega Enterprises, Ltd. |
| Type | 8-bit home video-game console |
| Released | October 1990 (Europe) 1990 (North America) 1991 (Brazil, TecToy) |
| Discontinued | 1996 (JP/NA/EU) • 2003 (Brazil) |
| Intro price | US US$99 (1990 pack-in) • UK £79.99 |
| CPU | Zilog Z80 (clone) @ 3.579545 MHz (NTSC) / 3.546894 MHz (PAL) |
| Memory | 8 KB Work RAM • 16 KB Video RAM |
| Storage | ROM cartridges (‘‘Mega Cartridge’’ 128 KB – 512 KB; a few 1 MB with mapper) Integrated ROM game or BIOS |
| Display | 256 × 192 (NTSC) / 256 × 224 (PAL) • 32 colours onscreen from 64-entry palette • 64 sprites |
| Sound | Texas Instruments SN76489 PSG (3 tone + 1 noise) • 8-bit mono |
| Dimensions | 225 mm W × 215 mm D × 70 mm H |
| Weight | ≈ 0.8 kg |
| OS / Firmware | None – game ROM executes directly |
| Predecessor | Sega Master System (Model 1) |
| Successor | Mega Drive / Genesis |
| Codename | MK-2000 / Model 3000 |
| Model no. | VA0 – VA1 motherboard revisions |
The Sega Master System II (SMS II) is a low-cost redesign of Sega’s 8-bit Master System, launched in 1990 to extend the life of the platform alongside the 16-bit Mega Drive. Sega’s engineers removed costly interfaces (card slot, A/V RGB, expansion port) and shrank the PCB to reduce bill-of-materials to under US$40, allowing aggressive pricing against Nintendo’s NES.
Internal Architecture
[edit | edit source]| Sub-system | Specification (Model 3000-07, PAL VA1) |
|---|---|
| CPU | Custom Sega 315-5685 (Z80 core) @ 3.546 MHz (PAL) |
| VDP | Sega 315-5246 (derived from Yamaha V9938) • 2 scroll planes, 64 sprites, 16 KB dual-ported VRAM |
| Audio | TI SN76489AN PSG clocked by VDP / 16 (≈ 357 kHz)
(Left & Right tied → mono output on RF/composite) |
| Memory | 8 KB SRAM @ 3.5 MHz (6116)
16 KB VRAM (2 × 8 KB TMS4416 or built-in VRAM in 315-5713 ASIC) |
| Bus I/O | Cartridge port 50-pin edge, I/O port (controllers), RF modulator, linear PSU |
Z80 Memory Map
[edit | edit source]| Range | Size | Function |
|---|---|---|
0000–BFFF |
48 KB | Cartridge ROM / BIOS; 8 KB per-page via mapper |
C000–DFFF |
8 KB | Work RAM |
E000–FFFF |
8 KB | Mirror of C000–DFFF |
FFFC–FFFF |
— | Bank-control registers (mapper) |
Boot priority:
- Internal BIOS (8 KB or 128 KB) – shows “SEGA” splash + built-in game.
- If BIOS disabled (JP pads), control jumps to cartridge vector
$0000.
Video Modes & Timing
[edit | edit source]| Region | Active area | Master clock | Lines/frame | Refresh |
|---|---|---|---|---|
| NTSC | 256 × 192 | 53.693175 MHz ÷15 = 3.579 MHz (VDP) | 262 | 59.922 Hz |
| PAL 50 Hz | 256 × 224 | 53.203424 MHz ÷15 = 3.5469 MHz | 313 | 49.701 Hz |
VDP can output composite and RGB; SMS II only routes composite + mono audio to an RF modulator, necessitating A/V modding for RGB.
Cartridge / Edge Bus (50-pin)
[edit | edit source]The SMS II retains the 50-pin “Mega Cartridge” connector used by the original console:
| Pin | Signal | Direction | Notes |
|---|---|---|---|
| 1 | GND | – | Ground |
| 2 | D7 | ⇄ | CPU data |
| 3 | D6 | ⇄ | … |
| … | … | … | … |
| 43 | /WR | ← | Controls SRAM writes on mapper carts |
| 44 | /MREQ | ← | Memory request |
| 45 | /M0-5 | ← | Slot select (low when cart area $0000–$BFFF accessed) |
| 46 | /IORQ | ← | Unused by ROM, active for My Card adapter |
| 47 | +5 V | — | 500 mA max |
| 48 | T_CE | ← | Sega mapper paging |
| 49 | RESET | → | Resets cartridge mapper / SRAM |
| 50 | GND | – |
Full table on Sega Master System Cartridge Pin-out.
Historical Context & Sales
[edit | edit source]- Announced July 1990 as a cost-reduced successor; included *Alex Kidd* or *Sonic the Hedgehog* in ROM to stimulate sales.
- Europe & Brazil remained the Master-System strongholds: by 1993 > 6 million SMS II units shipped in EU territories; TecToy sold an additional 2 million (1991-2003).
- In North America it served as a US$50 budget console beside the Genesis (approx. 300 k units).
Motherboard Revisions & Known Errata
[edit | edit source]| Rev | ASIC | Notable features / quirks |
|---|---|---|
| VA0 (1990) | 315-5246 VDP + discrete PSG | Separate SN76489; composite jail-bars; BIOS 1.3 (Alex Kidd) |
| VA1 (1992) | 315-5713 “One-Chip” | Integrates VDP + PSG + mapper; 64-pin QFP; some units have audio buzzing due to missing 10 µF cap at C31 |
- Early VA0 boards omit 33 Ω series resistors on RGB lines ⇒ over-driven when modded.*
Audio Path
[edit | edit source]Z80 → SN76489 square/noise → low-pass RC (33 k/3.3 nF) → CXA1145 composite encoder → RF can; stereo mods tap pre-RF and split PSG tone channels.
Common Faults (SMS II)
[edit | edit source]- No RF / distorted video – cracked solder on CXA1145 or corroded RF shield.
- Low audio volume / buzzing – dried C31 10 µF coupling cap (VA1).
- Power LED flicker – 5 V rail ripple; replace 2200 µF filter cap (C8).
- Controller port failures – broken traces to 315-5237 I/O decoder.
Detailed procedures: Master System II Troubleshooting Guide & Master System II Capacitor Replacement Guide.
Maintenance & Mods
[edit | edit source]- Composite/RGB A/V mod: lift RF modulator pin 11, route CXA1145 RGB → mini-DIN 8; add 75 Ω/220 µF caps. Step-by-step on SMS II AV Output Guide.
- Pause-button cap fix: add 100 nF across reset-pause lines to avoid random resets on VA0.
- FM-Sound board: installs YM2413 + logic; requires decoder ROM patch or EverDrive.
Trivia & Pop-culture
[edit | edit source]- European advertising used the slogan “The Leader of the 8-Bit Revolution” even after the Mega Drive launched.
- Brazilian TecToy versions included *Mônica no Castelo do Dragão* (a comic-book reskin of *Wonder Boy II*).
- An SMS II appears in Netflix series Stranger Things S4-E1, but incorrectly wired to a US CRT with PAL console shell.
See also / further reading