I have a G4/466 and would like my Yamaha CBX-T3 to work from it. The CBX-T3 has two means of connection:
1 a 'To Host' mini 8-pin serial port
2 a MIDI 5 pin DIN

I have looked for USB to MIDI convertors and can find none

Any ideas?

You don't 'convert' USB to midi, you must use a USB midi interface, every manufacturer has one (MOTU, emagic, midiman…)

There are usb to serial adapters, but it's not sure that the CBX is supported.

That's the idea

Thank-you - this was helpful. Armed with the right language ('interface' not convertor or adapter), I went to my local music shop and got the right part. I bought a Yamaha UX96 because it has a Macintosh serial 'TO HOST' socket.

At fifty UK pounds, it's a ridiculously priced object - my CBX-T3 only cost 100 new, and that came with Cubase software.! Nevertheless, it works after some grief and aggro - if you're not too MIDI savvy, some experimentation is required. It has occasionally lost the connection with the computer, but simply unplugging and replugging the USB fixed this.

Thanks again.