Mega CD/Sega CD Multibios
It is possible to turn your Sega Mega CD into a true multiregion machine. Arakon developed a MultiBios years ago.
http://arakon.dyndns.org/tutorial/bios.html
If you want to purchase the new Bios chip, you'll have to email that sites author, you should then receive the Bios, a socket for it and some switches (not shown).
Now available to download and program a chip yourself! I've got a copy of all of this and will host it if Arakons site ever vanishes. I've also got hold of a blank chip which I've programmed with the Multibios code, I haven't got a MegaCD to try it in yet though :(
I've just fitted one of these to a Mega CD 1 unit, the original front loader drawer piece of hardware. It works very well. Everything that you need to know is on Arakons site. Here's a bit of info/photos I made whilst fitting it which complements the original guide.
Remove the screws underneath the Mega CD and the 2 either side of the cart connector. Once open, remove the ribbon cables and 2 pieces of metal shielding. Also remove the connecting block that links the Mega CD to the Megadrive, just 2 screws (highlighted green) to undo and pull it sideways to remove it, finally remove the whole board.
Right next to the connectors is the original BIOS chip, because this is a MegaCD 1 we've got enough room to piggyback the new socket and chip on to this.
Start by bending the pins out on the 40 pin DIP socket, legs 20, 38 and 39.
I found this bit the most difficult, be very careful doing this, it's easy to damage the board and/or any of the nearby components. You have to lift up leg 20 of the ORIGINAL BIOS, I used a pair of side cutters to cut the leg as close as possible to the board. Now straighten the leg so that it's parallel with the board.
Right the rest is quite easy, I really didn't like cutting leg 20! Test fit the socket over the old chip, then solder all the legs (except 20, 38 and 39 which you bent out earlier). Work around each leg with the soldering iron, carefully heating the solder on the old leg up, and dragging the iron up and over the leg of the new socket, if necessary add a tiny bit more solder, but be careful, you don't want to add too much and accidently solder adjacent legs together.
You have to link up leg 20 of the ORIGINAL BIOS to leg 1 (remember leg 1 is now soldered to leg 1 of the socket)
Another link wire now. You need to connect leg 20 of the SOCKET to the solder pad which leg 20 of the original BIOS was attached to. It's much easier to do this by soldering the wire to the spot on the underside of the board and then to socket leg 20.
Prepare you switches, I started by adding solder to each of the contacts on the switches and then linking up the Ground wires. It's not very clear in the photo but the black wire with white bits goes to the left contact on the left switch, and then goes over to the left contact on the right switch. The power signal +5v is the purple wire, it goes to the right contacts of both switches. Finally the middle contacts, I've put a green wire on one switch this will link to leg 38 of the socket, the other switch has a white wire which will link to leg 39 of the socket.
The next couple of photos show the new BIOS chip in the socket, I haven't said to install it yet, I just took the photos in the wrong order.
The photo above shows the Purple wire (I promise you it is purple!) linked to leg 1 (where you soldered a link wire earlier).
And the photo above shows the Black (with white) wire going to leg 30 , Green soldered to the bent leg 38 of the socket and White to the bent leg 39 of the socket.
Now that you've done all of the above, you're ready to test it out. By flicking the switches you have 4 different selections that will send either +5v or GND to legs 38 and 39, which will in turn select the BIOS from the chip.
| JAPAN | USA | EURO | GAME | |
| Leg 38 | GND | +5v | GND | +5v |
| Leg 39 | GND | GND | +5v | +5v |
Because of the fact there are 3 regions but 4 possible positions Arakon included a bonus game as the fourth option. Arakon also removed the region check on the BIOS files, so you don't even have to mod your Megadrive (although it is recommended and easy). You cannot leave legs 38, 39 floating, they must be connected to ground or +5v for the chip to work, I tried it and just got a white screen.
I do want to try using a single 3 position switch to select between the 3 different regions (ignoring the bonus game), I haven't tried it yet. I did ask around here and here about it if you wanted to try it out. I also found out you can get on-on-on switches, and I drew a diagram to try out, I think it will work.
Console Passion recently supplied me with an On-Off-On switch wired up to select between the 3 different regions BIOS. It's simple to wire up and needs just a Single Pole/Double Throw Centre Off Switch and 2 1k Resistors.
The photos here show a double pole switch (two rows of contacts), but you only need a single pole switch with one row of contacts. Start by soldering the wrapping a leg of resistor around one of the end switch contacts, then solder it on.
Here you can see that I've added the other 1K resistor to the other end leg, joined them together and then soldered the black (Ground) wire to the join (the other end is linked to leg 30 of the BIOS.
The White (leg 39) and Green (leg 38) wires are soldered to the switch contacts with resistors on them. The Purple wire is 5v and is soldered to the middle switch contact and the other end to Leg 1
Thanks again to Console Passion (and also Paul Willingham) for confirming how to wire up the On-Off-On switch :)
Summary of Components needed
1 x MultiBios (this is
Arakons code on either a 27C4096 or 27C4002 EPROM)
2 x Single Pole, Double Throw switches
1 x 40 pin DIP socket
some wire
I used a Willem Parallel Port Programmer, with a 16-bit adapter and a 12 volt PSU to put the code on the chip.
