that did it!!! thanks so much, my head was going to burst into little pixels... Just in case someone makes the same mistake I did... I was connecting the ground pin on the optoisolator to the ground on the gb (while using another power source) ... duh! !!!!!

The most important things for midi are:
- The diode - if its in the wrong way, Midi wont work
- The Midi cable / jack ... try flipping the 2 wires
- The optoisolator chip - Check that its receiving power & ground and that all the pins are going to their right place
- The arduino - Does it do the startup sequence of leds? If so then everything should be fine with that

That should do it.

thanks for both replies, I've checked both of the possible solutions and then i found out arduinoboy is not flashing the light's when MIDI data is beig sent to the corresponding channel, what is the possible trouble shooting for this? I've double checked the schematics and i'm quite sure everything is in it's correct place, I'm also quite sure all resistors and the diode are fine, is it possible the optoisolator is messed up? or maybe eveb the arduino board? how can i check this?

in mGB mode each led should blink in response to a midi message sent on its corresponding midi channel.
IE: if you send a note on channel 1, 1 led should blink, channel 2 the led next to that led should blink etc.

if that is working, then the arduinoboy is receiving midi just fine- so it would be a problem with your gameboy cable wiring. check the schematic's instructions and use a multimeter or wire tester from the end of the gameboy cable to the arduino to verify everything is done right.... if it still looks fine, check the schematic's pin output again for the cable, might have it backwards or something.

Technically there are only 2 wires and ground needed for things to work for receiving midi messages: Clock (arduino ANALOG IN pin 0) and Data in (arduino ANALOG IN pin 1)

Looking at the end of the gameboy cable...

/ [1] [2] [3] \
| [4] [5] [6] |

1: +5volts power
2: Data in
3: Clock
4: Data out
5: not used
6: Ground (you can also use the shielded ground from the cable)

did you set the axiom to do midi out? i had the same problem (maybe)

hit the advanced button, and then midi out (first B)

I've been hooking up an arduinoboy using a RBBB Kit from Modern Device, USB BUB Board as a powersupply and a M-Audio Axiom25 as a MIDI Controller. All the leds light up in the correct order, Mode Switch also works perfectly, I'm also sure MIDI messages are being send in Nanoloop Mode and LSDJ Master Mode since the led for logical level in the BUB Board lights up everytime I hit a key on the keyboard, but I can't get it to do anything, in one of my first tests i was able to play a note on mGB but that's all, could someone give me a hand, could it be something i have to set on the MIDI Controller? is the Arduino board toasted? Should the status led light up only at start-up? Please help me out!!!