ST-V (SEGA Titan Video) Dual BIOS

Introduction

This project is for owners of a SEGA ST-V (SEGA Titan Video) PCB.

The ST-V is a cart based system which allows you to change game by just inserting another game cartridge. All cartridges are the same physically. There is no keyed connector protection like on the Taito F3 motherboard, where a Japanese cart will only fit a Japanese motherboard.

On the ST-V it's the BIOS on the motherboard which controls the region selection. The following types of cart exist:

• Carts which work with all regions: Most carts.
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• Carts which work with all regions and change their in-game text according to the region selected: Soukyugurentai/Terra Diver is playable on all regions but in addition it will change the in-game text to English with an "English" BIOS.
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• Carts which work with all regions and change their gameplay depending on the region selected: Radiant Silvergun will only use two buttons if you play it with an USA or European BIOS!
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• Carts which contain the same game (albeit with different language) but which are locked to a specific region: Dynamite Deka and Die Hard Arcade are the same game, however Dynamite Deka will only work with a Japanese BIOS and Die Hard Arcade will only work with an "English" BIOS.
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• Carts which are locked to one region: Puyo Puyo Sun will only work with a Japanese BIOS. However Columns 97 will work with all BIOS's except USA!

Instead of buying an extra motherboard to make sure you can play all your games in the language you desire, you can just change the BIOS on the PCB.

Region selection

Inside each game cart there is region string. The region string consists of 4 characters. The possible characters are:

• 'J' - Japan
• 'U' - USA
• 'E' - Europe
• 'T' - Taiwan

Inside the BIOS there is a region character (offset 0x809). It contains the letter 'J', 'U', 'E' or 'T'. When the PCB is powered the BIOS verifies that the region character can be found in the region string stored in the cart. Failure to find this will prevent the game from starting and a suitable error message will be displayed.

Changing the BIOS

One solution is to burn a copy of an alternate BIOS into a 27C400 EPROM. To save wear and tear on the chip socket you may want to buy a ZIF (Zero Insertion Force) socket as well.

An even simpler solution is to burn two ROM images into an EPROM and then use a technique called bank switching to select between the two of them.

Note:
If your ST-V mother board is equipped with an EPROM of type 27C4096 (or equivalent) you cannot make a dual BIOS. The reason for this is that the 4 Mbit (256K x 16-Bit) EPROM is not available in 8Mbit. The only solution is to buy a ZIF socket, burn the alternate BIOS image into another 27C4096 and switch manually.

Bank Switching

The ST-V BIOS image is 524288 (0x80000) bytes long. This means that one needs 18 address lines to address the entire image. If one has 19 address lines one can address 1048576 (0x100000) bytes or twice the amount of memory.

The following drawing shows two images which are stored in memory of a 27C800 EPROM. This device is capable of storing 1Mbit or 1048576 bytes of data.

By using 18 address lines we can address up to 0x080000 i.e the image. By using 19 address lines and setting the topmost bit (A18) to 0 we can still only address 0x080000 bytes. If we set the topmost bit (A18) to 1 we will start accessing the image and this image only!

This technique is called bank switching. By changing the address of the upper address bit we will effectivly switch banks. By storing two BIOS images in a 27C800 EPROM we can switch between them by setting address line A18 high or low.

So in order to get a dual BIOS all you need to do is to concatenate two BIOS images, burn them to a suitable EPROM and install a simple switch.

To determine which BIOS images to burn you might want to compare the list below against the games you own.

ST-V Gamelist

Here is a list of ST-V games and which region they will operate in. This list is not a complete list of all ST-V games.

The only game on this list which requires an USA BIOS is "Super Major League".

Requirements

• 3 wires preferably of different colour.
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• A slide switch of type Single Pole Double Throw or SPDT.
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• Some double sided tape or glue to make the switch stick to the EPROM.
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• A 16 bit 1 MBit EPROM of type M27C800.
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• Soldering equipment.
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• A ROM Burner which can burn a 27C800 EPROM.
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• Two different ST-V BIOS ROM images (preferably Japan and Europe). The USA version will display the "Winners don't use drugs logo".

  Note:
  Do not email me asking where to find these images. All such mails will be deleted. Use your favourite search engine instead.

• Some knowledge of basic DOS/WIN/UN*X commands.

Disclaimer

I take no responsibility whatsoever for any damage you might cause to your hardware, yourself or your surroundings when making this BIOS.

If you don't have the expertise to make your own dual BIOS, you can order one from JammaBoards.

Note:
I do not sell or manufacture these ST-V Dual BIOS's. So please don't ask me to make or sell you one.

BIOS Images

Here is a list of all the known ST-V BIOS images and their CRC32's:

Procedure

• For sake of argument we will make a dual Japan/Euro BIOS.
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• If you need to read your own BIOS you can read it as a 27C400.
  
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• From DOS or a DOS/CMD window issue the following command:
  
  copy /b mp17951a.s+mp17954a.s dual_stv.bin
  
  From UN*X the command will be:
  
  cat mp17951a.s mp17954a.s > dual_stv.bin
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• This will put the Japanese BIOS in the bank and the European BIOS in the bank.
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• Start up your ROM burner and burn the dual_stv.bin image into a 27C800 EPROM.
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• Solder your coloured wires to pins 22,31 and 1 like in the illustration below. I would suggest a red wire from pin 22 and a black one from 31. Choose your favourite colour for the wire from pin 1. Make sure the wires are long enough to fasten the switch to the top of the EPROM (between pins 1 & 42).

  

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• Put a label on the EPROM window to prevent accidental erasure and fasten the switch like in the image below.

  

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• Remove your MASK BIOS chip from the ST-V motherboard. If you don't have a proper EPROM extractor use a small screwdriver.
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• Insert the dual ST-V BIOS like in the picture below.

  Note:
  Pins 1 & 42 are supposed to be outside the socket!

  

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• When the switch connects the black & gray wires, the Japanese image will be selected whilst red & gray will select the European image.
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• Insert your Soukyugurentai cart, flip the switch to "European" and watch Soukyugurentai start as Terra Diver.

"The future of ST-V rests upon our work and your work"
(Text found inside BIOS image)

Email address: