how can I connect psr520 to my computer? Which cable I should use?

I assume you're talking about midi..... the easiest way would be a cable like this http://zzounds.com/love.music?p=p.THKTW12MC1&z=1000177054788 it has a midi in and a midi out port and hooks to the joystick port on your computer..... there are many cables like this out there, any of them should do fine.

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