Sega Genesis

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The Sega Megadrive, or Sega Genesis as it was known in North America, was Sega’s 16 bit console. Designed around a Motoral 68000 and Zilog Z80 pairing, which was very common at the time, the console had an elegant yet straightforward architecture which made it very easy for programmers to exploit its power.

SMS Mode

The Sega Genesis is natively backwards compatible with the Sega Master System. In fact, most of the hardware contained in the Master System is also found in a Genesis: Z80 processor, PSG, etc…
To enter Master System compatibility mode, it seems that you must, at the very least, do the following (refer to the Pinouts section for Genesis pin locations):
 Genesis Pin  Name  Connect to
 B30  #M3  GND
 B9  A21  Vcc
 B10  A22  GND
With the above connections, the cartridge port loses the #RESET signal on B27 – it now constantly outputs GND instead of a nice active-low reset pulse when the reset button is pressed. Therefore, you cannot rely on B27 for reset when in Master System compatibility mode. This is the source of much frustration for me! The cartridge can still send a reset signal to the console via pin B2 in Master System mode.

 

Pin Reconfiguration

Other Genesis pins change functions when entering Master System compatibility mode. There is very little information available on these, the information listed below was found on SMSPower.org and through experimentation. The list is not exhaustive and I will keep adding to it as I discover more…
 Genesis Pin  Genesis Name  SMS Function
 B28  #LDSW  #WR
 B17  #CE  #M0-7
 B16  #OE  #RD
 B11  A23  #PAUSE
 B6  A18  #CE
 B7  A19  #IORQ
 B27  #RESET  GND

Power

Console Current Consumption

These values were measured on NTSC consoles using a 9V benchtop power supply. These values are averages because the actual current consumption is dynamic – though according to my experiments they don’t appear to vary much.
 Unit  Avg (mA)  Peak (mA)  Notes
 Model 1 HD Graphics  700  700  Measured with and without a game inserted (Sonic 2), didn’t seem to make a difference.
 Model 1 Sega CD  500  1000  Peak occurs right after booting. The Sega CD does not power up if a Genesis cartridge is inserted.
 Model 2 Sega CD  500  1000  Peak occurs right after booting. The Sega CD does not power up if a Genesis cartridge is inserted.
 Model 2 Genesis  300  300  Measured with and without a game inserted (Sonic 2), didn’t seem to make a difference.
 32X with 32X game  350  350  Measured with Doom
 32X with Genesis game  200  200  Measured with Sonic 2

Game Cart Current Consumption

These values were measured on an NTSC Model 2 Genesis. These values are averages because the actual current consumption of cartridges is dynamic.
 Game  Avg (mA)
 Sonic The Hedgehog 3  13.2
 Sonic The Hedgehog 3 and Knuckles  11.9
 Super Street Fighter 2  19.1
 Virtua Racing  120.4
 Mega Everdrive  133.1 (160+ while loading)
 Power Base Mini + Double Dragon  6.9
 Power Base FM + Double Dragon  48.6

Power Supply Connector Type

 Unit  Connector Type  Polarity
 Model 1 Genesis  2.1mm ID x 5.5mm OD  Center negative
 Model 1 Sega CD  2.1mm ID x 5.5mm OD  Center negative
 Model 2 Sega CD  2.1mm ID x 5.5mm OD  Center negative
 Model 2 Genesis  EIAJ-03 (1.7mm ID x 4.75mm OD)*  Center positive
 32X  EIAJ-03 (1.7mm ID x 4.75mm OD)*  Center positive

*Some distributors will list the Model 2 Genesis and 32X power supply connector as 1.7mm ID x 4.8mm OD

Schematics

Model 1

Model 2 VA0

Model 2 VA1

Documents

René

Electronics engineer and retrogaming fanatic!