Alan,
I agree with Phil's assessment that the Yamaha interface probably does not include a Generic MIDI path, although it would probably be worth your while to check with Yamaha to be certain.
If that is the case, your options are 1) the serial interface that you mentioned ( or another serial interface, as there were many varieties out there at one point); or 2) a parallel port interface, like those made by Mark of the Unicorn (MOTU) and Opcode/Music Quest.
On Option 2: I used a Music Quest 'Note1' parallel-port adapter with Win98 for several years. It worked well, and the only complaint I could come up with is that it is not plug-and-play; the Note1 must be connected to the parallel port when you boot into Windows, or it isn't recognized. I only paid about $85US for a new one, so I would suspect that a used model would fetch considerably less than 80 pounds. (I sold mine for about $30US.) The downside is that Opcode no longer even supports the product. If you end up going this route, keep a spare copy of the drivers in a safe place.
I've also heard good things about the MOTU MIDI Express/XT, but it is considerably more expensive, as it's an 8x8 patchbay (with SMPTE?). I believe that they have a half-rack 4x4 equivalent, too.
Regards,
-BW
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Bruce Wahler
Ashby Solutions™
http://music.ashbysolutions.comCloneWheel Support Group moderator
978.386.7389 voice/fax
bruce@...