Specifications

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Communication Overview
21
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20Mbps. The 2.1 spec. does not provide for bus mastering,
DMA, or multiple interrupts, (however, Quatech's interrupt sharing
software drivers allow sharing the one interrupt among multiple
I/O devices). While PCMCIA provides only a minimal performance
improvement over ISA, and does not come close in speed to PCI,
it does provide for considerably more flexibility than either of
the others.
The two most important features of PCMCIA are its Plug and
Play and Hot Swapping capabilities. As with PCI, PCMCIA cards
are truly Plug and Play--you simply insert them, and instructions
coded into chips on the card provide the information a host
needs to configure the cards and appropriately allocate resources.
Not only are there no jumpers or switches to set, users never
even see the inside of a PCMCIA card. It is simply inserted into
the drive, and the system does the rest. (An open PCMCIA card
is pictured below, to show what you've been missing.)
This configuration procedure, along with the fact that PCMCIA
cards are not connected directly to the motherboard, but are
easily inserted into and ejected from a PCMCIA drive, allows the
cards to be hot swappable. This means that the system need not
be shut down then re-booted to add, remove, or exchange cards.
Thus, you could insert a PCMCIA scanner, scan a drawing of
your newest board layout, then remove the scanner and insert a
modem and e-mail the scan to a manufacturer for mass
production. While this might not be very important for desktop
PCs with large numbers of expansion slots, it is vitally important
for laptops with limited resources and usually only two PCMCIA
slots. It becomes even more important for hand-held computers
which often have only one PCMCIA slot and one serial port.
32-Bit CardBus
In 1995 the PCMCIA 2.1 specification was enhanced to provide
for 32-bit operation. The new architecture, called CardBus , was
closely based on the PCI bus, and strove to provide the same
improvements over the 16-bit PCMCIA card as PCI did over ISA.
As such, CardBus provides for 33MHz operation and
correspondingly increased data transfer. It also introduces DMA
and bus mastering to PCMCIA based systems, which can markedly
increase performance. Realizing that there are still many 16-bit
PCMCIA card peripherals in the marketplace CardBus is fully
backward compatible with the older card design.
Because of this backward compatibility, Quatech has decided
not to redesign our serial data communication PCMCIA cards for
CardBus, as doing so would limit the number of systems that
could use our cards. As discussed with PCI, because of the
limitations imposed by serial and parallel transfers, there would
be no noticeable performance gains for Quatech serial cards under
CardBus. However, our new DFP-100 two port IEEE1394 card,
(see page 57 for card specifications and page 24 for more on
IEEE 1394) does use the CardBus interface. IEEE 1394 peripherals
can communicate at speeds up to 400 Mbits/sec, and
thus can take advantage of the higher speed
communication CardBus provides.
PCMCIA for Data Communication
Though PCMCIA card use is not limited to portable
computers, (see pages 58-60 for PCMCIA drives for desktop PCs)
there are few instances where it is the best choice for data
communication in desktop computers. In desktops, PCMCIA is
better suited for adding extra storage space via hard-disk cards,
or transferring large files from portable systems. However, for
laptop and hand-held computers, PCMCIA provides a way to
connect a varied array of peripherals to the system, and to share
those devices with a desktop computer.
Clearly there is a size advantage to PCMCIA for portable
applications. The cards are small, light, and have low power
requirements. They are an ideal interface choice for peripherals
that have been scaled down for portable use. Further, the ability
to Hot Swap PCMCIA cards provides for the flexibility needed to
use multiple peripherals with only one or two slots. USB and
IEEE 1394, which also provide Hot Swapping, (see pages 22-
24), are other alternatives for portable applications. IEEE 1394
uses a particularly small cable ideal for portable applications,
and is ideal for high speed audio and video applications. However,
to use USB or IEEE 1394, the peripheral devices in your system
must be replaced with bus specific devices--an expensive
prospect. Further, many USB products must be powered by the
computer itself, thereby reducing the time a laptop can function
on battery alone. Or, if too much power is required, they must
be plugged-in, making them less attractive portable solutions.
So if cost and power conservation are your primary concerns,
PCMCIA is still the best, most flexible choice for portable
applications.
(Quatech's QSP-100 PCMCIA Card Uncovered)