Thanks to Sean Smith (Las Vegas - Nevada) for
the following information :
I wanted to share some reasearch i have done on the american Sega Genesis,
I believe this may apply to the Mega Drives as well.
There were quite a bit of revisions of models, The early model in 1989
could play unlicenced titles because it didn't have a lockout chip, and
read High Definition Graphics on the top of the ring and had an external
port on the back for a modem that was only released in japan. It also
had better audio clarity.
Then in 1990 they added the lockout chip while still keeping the external
port on the back and the High Definiton logo still on the top of the ring,
this revision still has clear audio.
Then in 1992 Sega decided to take out the external port, remove the High
Definition Graphics logo and make the red part of the plastic around the
light a seperate piece of plastic, before it was painted on the ring.
This models audio was fuzzy sounding and the contacs on the expansion
port for the sega CD were not silver anymore; they put cheap black material
for the contacs.
There is a model bettween 1990 and 1992 revisions that had the High Definition
Graphics logo removed and the external port removed but inside the hardware
is the same.
When 1992 came around they stoped using the big Motorolla processor and
used a cheap set of clone chips.
That concludes all the revisions of the model 1 Genesis. The best one
to get is the 1989 one. You get the full quality of what you would have
payed for in 1989. There are distinctive factors that tell you the differance
when shopping for one, first of all you will want to look for one with
the "HIGH DEFINITION GRAPHICS" logo on the top of the ring or
"High AV Terminal" for the japanese mega drive. Then to tell
if it is the 1989 model with out the lockout chip instead of the 1990
model with it, you want to look at the front rubber boots on the botom
that support the system. The 1989 model will have real rubber boots and
the 1990 model and later will have plastic. Good luck and have fun, If
you find one of these you have to get the Sega CD Model 1 for it, they
also have better audio clarity and generally look better with the Model
Epicenter (USA) sends us some corrections concerning
the text above :
Just wanted to correct some inaccuracies in the original post.
#1. The Megadrive never had a lockout chip. The glue chip had a new BIOS
incorporated into it in a secondary revision adding the copy protection
.. also the VDP Then required SEGA be written into it before it would
#2. The MC68HC000 (smaller surface mount 68000) or Hitachi HD68HC000
weren't 'cheaper clones'. The MC68HC000 is a surface mount variant from
Motorola. Hitachi's HD68HC000 is a licensed clone and actually performs
MORE work per clock tick than the real McCoy from Motorola under numerous
tests I've performed. The only weaker clone was the Signetics SCN68000
used in the VERY FIRST versions of the MegaDrive model 1, which did FAR
LESS work per clock tick, about 20% less!
#3. I am unable to confirm claims of any lower sound quality on newer
MegaDrive revisions except the Genesis 3 released by Majesco which omitted
the Z80 and incorporated a workalike into a large combination chip.
#4. The EXT port acted as a controller port and is wired into the glue
chip as such. It was used to select a game on a second display in the
MegaTech/MegaPlay arcade system. It was removed from the design spec shortly
after the first revisions at the same time as HIGH DEFINITION GRAPHICS
was removed from the lid.
I may be wrong about this but I believe the modem released in Japan utilized
the cartridge slot or Mega CD / Demo Unit edge connector rather than this
port. However the Megaplay/Tech application appears to be the intended
purpose of this port.
For all interested, the MegaDrive/Genesis overclocking page is here: