card. You can
put any function onto a PC card that a desktop slot of the
corresponding type would have supported (and, conversely, you cannot, for
example, create a 16-bit PC card USB adapter with USB ports; the necessary
resources and data rates don't exist on the ISA bus, a PCI slot is
required).
I have personally seen a 16-bit USB PCMCIA card, which are much sought after
by older Mac laptop owners who do not have CardBus (such as 1400s, 5300s,
etc.). They are practically unobtanium, but they did exist.
As did ISA USB cards, for that matter.
Peace... Sridhar