Editing Sega Master System II

{{Infobox computer
| name         = Sega Master System II
| image        = [[File:Sega-Master-System-II.jpg|250px]]
| caption      = European PAL “SMS II” (Model 3000-07) with integrated ''Alex Kidd in Miracle World''
| manufacturer = Sega Enterprises, Ltd.
| type         = 8-bit home video-game console
| release date = October 1990 (Europe)<br />1990 (North America)<br />1991 (Brazil, TecToy)
| discontinued = 1996 (JP/NA/EU) • 2003 (Brazil)
| price        = US US$99 (1990 pack-in) • UK £79.99
| cpu          = Zilog Z80 (clone) @ 3.579545 MHz (NTSC) / 3.546894 MHz (PAL) <!-- master ÷15 -->
| memory       = 8 KB Work RAM • 16 KB Video RAM
| storage      = ROM cartridges (‘‘Mega Cartridge’’ 128 KB – 512 KB; a few 1 MB with mapper)<br />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           = None – game ROM executes directly
| predecessor  = [[Sega Master System]] (Model 1)
| successor    = [[Sega Mega Drive – Model 1|Mega Drive / Genesis]]
| codename     = '''MK-2000''' / '''Model 3000'''
| model        = 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 ==
{| class="wikitable styled-table" style="width:90%; text-align:center;"
! 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 ==
{| class="wikitable styled-table" style="width:80%;"
|+'''SMS II Address Space'''
! Range !! Size !! Function
|-
| <code>0000–BFFF</code> || 48 KB || '''Cartridge ROM / BIOS'''; 8 KB per-page via mapper
|-
| <code>C000–DFFF</code> || 8 KB  || '''Work RAM'''
|-
| <code>E000–FFFF</code> || 8 KB  || Mirror of C000–DFFF
|-
| <code>FFFC–FFFF</code> || — || 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 <code>$0000</code>.

== 📺 Video Modes & Timing ==
{| class="wikitable styled-table" style="width:70%; text-align:center;"
! 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:

{| class="wikitable styled-table" style="width:90%;"
|+'''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 [[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 <small>(approx. 300 k units)</small>.

== 🛠️ Motherboard Revisions & Known Errata ==
{| class="wikitable styled-table" style="width:85%; text-align:center;"
! 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]]

[[Category:Sega Systems]]