I'll add that the dual-cassette model's purpose is exactly the same as the floppy drive unit's--to take the place of a paper tape punch and reader. You can still find these boxes attached to older commercial embroidery machines. So they should respond exactly the same way as does a paper tape unit, namely X-ON, X-OFF etc.
The more interesting aspect is that these floppy boxes are turning up with Gotek (or similar) floppy emulators in them.