Adding a Link Port to Super Gameboy

Adding a Link Port to Super Gameboy - Joshua Neal (V.1 10/16/1999)

Warning: This is the documentation of the my own project. There is the potential to damage your hardware, so if you attempt to do this, know what you are doing before attempting this. I specifically disclaim any responsibility for injury or damages caused.

The Idea

The idea came from a web page in Japanese, so the original credit goes to the authors of that page, Papa and Shaora, as they are the ones who figured out that it would work. That page is at the following URL, you may want to check it out for the pictures:

http://www.bomber.co.jp/chaola/labo/SGB_comm/ (Dead link, editor's note)

Theory

Basically the SGB-CPU in the Super Gameboy is the same as the one in the gameboy itself, including the pinout. This means that the hardware for link port communications exists, it just isn't connected to anything. Nintendo makes a Super Gameboy 2 for the famicon that has the link port. This would have cost me a bunch of $$$, so I chose to try to add one to my original SGB.

Inside the SGB

The first task was opening the Super Gameboy. This required a special tool that I don't have. After attempting futiley to remove them without that tool, I took the game to a local used game store and asked them to open it for me. They thought it was strange, but they did it anyways...

Comparing the circuit board inside with the pictures on the japanese page, I determined that the circuit board of the Famicon version of the SGB was identical to the Super Nintendo version. I located the three pins on the circuit board that would have to be soldered to, on the chip marked SGB-CPU.

These pins were really tiny, so I decided I needed some soldering practice. I found a non-working PC device with a chip that had the same pin spacing, and practiced soldering leads to that until I felt confident enough to do it on the SGB.

I found it worked best if I scraped the pin with a knife to remove any coating, put a small bit of solder on the pin, then prepare the wire and reflow the solder on the pin and stick the wire on.

Schematic

The schematic on the Japanese page used only a resistor for each data line, comparing Jeff Frohwein's GB and SGB schematics, it looked like an additional pair of diodes and a capacitor for each data line would provide the same protection and noise filtering as the gameboy. I decided to include these in my project. The schematics are located at:

http://home.hiwaay.net/~jfrohwei/gameboy/gameboy1.gif -- Gameboy Schematic
http://home.hiwaay.net/~jfrohwei/gameboy/sgb.gif -- SGB Schematic

Parts

• 7 germanium diodes
• 3 100 picofarad ceramic capacitors
• 3 220 ohm resistors
• spool 30ga wire-wrap wire
• 1 gameboy link port connector (female)

I used wire-wrap wire to make all the connections, only with solder joints instead of wire wrapping.

The japanese page shows using a link cable cut in half for the connector, while I had one of these lying around, I used a female port from an adapter that came with a link cable that would allow it to connect to a classic gameboy with the larger link port size. This was inside a nice black connector shell, that was held together with a small screw. Once opened, it turned out to be possible to cut the larger connector off and have a nice female port that would accomodate a standard link cable.

I used this connector in my project, also using the black shell, which I epoxied inside the SGB after cutting a matching hole in the side, leaving it such that I could still unscrew and open the connector shell. This made the connector sturdy, and look somewhat attractive as well.

Schematic

I soldered together a point-to-point circuit for this, roughly in the shape of the following diagram. It might have also been possible to make a small circuit board that fits inside the SGB. With this circuit, operation should be identical to that of the gameboy.

Key:

-|<-    germanium diode
-)|-    100pF ceramic capacitor
-/\/\/- 220 ohm resistor

VCC ----+--------------------+-------------------+---->|----- to link pin1
        |                    |                   |         
        +-|<-+--o     to     +-|<-+--o           +-|<-+--o 
 SGB         |        SGB         +       SGB         +    
 pin o-/\/\/-+  to    pin o-/\/\/-+ to    pin o-/\/\/-+ to 
 68          | link   69          | link  70          | link
        +-)|-+ pin5          +-)|-+ pin3         +-)|-+ pin2
        |    |               |    |              |    | 
        +->|-+               +->|-+              +->|-+ 
        |                    |                   |
GND ----+--------------------+-------------------+----------- to link pin6 

Link Port Pins (looking at external female connector)

 _____
/     \
|5 3 1|
|6 4 2|
+-----+

The VCC lead I soldered to a plated-thru hole in the SGB circuit board near the 5v pins (27 and 58) of the snes connector. The ground lead I soldered to a plated-thru hole in the large ground plane area.

Assembly

Once I got this circuit built and connected to all the necessary pins, I hotglued down anything that might shift around, cleaned up my soldering mess with flux remover, and put the SGB back together, only using screws with phillips heads instead of the original screws.

Testing

I made several picture transfers with a gameboy camera, and tested operation of the printer, all of which worked. I deemed the project a success!

- Joshua Neal <tinara@goodnet.com>