You are visitor number since November 28, 2001.
I have also been contacted by a large number of people interested in a design for a USB midi interface. Unless you are very clever with programming and have lots of experience with electronics and very detailed soldering I suggest you don't bother. Since I got myself a G3 Mac Powerbook which only has USB (no older style RS422) I have thought about and investigated this quite a bit. In short, the protocols required for USB mean that you would need at least a dedicated USB interface chip and maybe some memory and a microprocessor - WAY outa my league! If you are still interested, have a look at Thorsten Klose's fantastic MIDI DIY site. He has a design there (as well as a midi-merge circuit!) but be aware there are some limitations. I resorted to buying one. I got a Mark Of The Unicorn Fastlane 2x2 USB interface and it works quite well. Midiman, Opcode, Emagic and Roland also make them. For desktop G3s and G4s it may be possible to install one of the serial port cards such as the Stealth card or one of the MegaWolf cards and use my serial interface but I haven't tried this and it is probably more expensive than buying a USB interface anyway. It is not, to my knowledge, possible to use this type of solution with laptops. USB to Mac serial converters may work but I think it would be tricky. I have not tried them.
I started out with a Mac so I built myself a port-powered interface for that platform first (sometime in 1996 I think it was). Actually, that's not entirely true. I had an Atari 1040STFM before that but it had its own midi interface so I didn't need to build one. Then in 1998 my Mac got some company on the desk (the poor old Atari had long since become a hand-me-down to one of my brothers)....a Pentium PC! So far, they haven't started this sort of behaviour...
It soon became obvious that a midi interface for my PC would be handy as well so I got out my soldering iron again.
Both these interfaces worked so well, and were so simple to build and cheap compared with commercially available interfaces, that I decided to make the information available to everyone. Many people have already successfully built the Mac design and I am grateful for all the feedback I have received over the last couple of years. Now there is a PC version as well.
I apologise to all those folks with platforms other than Mac or PC (or Atari for that matter) for not having a circuit for you. You might be able to find something suitable at Harmony Central's interface page. Geoff Smith has designed a Midi interface for the Palm Pilot. If you can't find the one you want, don't be put off. Design one yourself! If you do manage to do this successfully, and you don't have a way of making the information freely available, let me know and I might be able to add your design here.
Anyway, here they are......
You could also try a search on "midi projects" or similar in the keyword search box below.