Sega Master System II

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

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

SMS II Address Space

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:

1. Internal BIOS (8 KB or 128 KB) – shows “SEGA” splash + built-in game.
2. If BIOS disabled (JP pads), control jumps to cartridge vector $0000.

📺 Video Modes & Timing

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)

The SMS II retains the 50-pin “Mega Cartridge” connector used by the original console:

Selected Pins

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

• 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

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

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)

• 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

• 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

• 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

• Master System II Maintenance Guide
• Master System II Troubleshooting Guide
• Master System II Capacitor Replacement Guide
• SMS II AV Output Guide