Firewire, USB, AND SCSI : DETERMINING THE INTERFACE THAT’S BEST FOR YOU
by Steve Becker

If you’ve recently purchased a new Mac, then you already have a
Firewire-equipped Mac; if you own an older Mac that doesn’t include a
Firewire port, you may be able to add Firewire to it by installing a
Firewire PCI card. In this article, I’ll discuss the pros and cons of
using Firewire with your computer, describe how Firewire compares to USB
and SCSI, and try to dispel some common myths/misinformation about
Firewire.

Firewire is designed to allow you to connect various types of hardware
(i.e., hard drives, scanners, video equipment, CD-RW drives, etc.) to
your Mac; in the case of drives, this includes either internal or
external devices. To appreciate Apple’s Firewire technology, you need to
understand the alternatives to it that are available.

THE SCSI INTERFACE

For many years, the SCSI (Small Computer Systems Interface) has been the
standard interface used for connecting devices to your Mac. The benefits
of using this interface standard include its having been widely used for
many years, so most types of hardware are available with a built-in SCSI
port. Also, over the years the SCSI interface has evolved, and it now is
available in several configurations, including ultra high speed up to 160
MB/sec (megabytes per second).

The downside to using SCSI-based devices include that this interface can
be highly temperamental (i.e., devices refuse to mount, disk errors
occur, strange system problems occur, etc.), no more than 7-15 devices
can be connected together in a single SCSI chain, you must assign each
device a distinct SCSI ID number before adding it to your SCSI chain, the
last device in the chain must be terminated, and all the SCSI devices in
the chain must be turned off (along with your computer) before a SCSI
device may be added to or removed from the SCSI chain (i.e., SCSI doesn’t
support plug and play/hot swapping of devices). Additionally, you must
be careful to turn on all your external SCSI devices (and, when
appropriate, allow them to spin up to speed) before turning your computer
on, and you must remember to shutdown your computer before turning these
devices off. Also, you need to use thick, relatively expensive, and
heavily shielded cables when connecting SCSI devices to each other and to
your Mac.

Basically, the SCSI interface offers both a large selection of devices to
choose from and is capable of high speed performance, but it can be a
real pain to work with.

THE USB INTERFACE

Several years ago, Apple decided to add a USB (Universal Serial Bus) port
to the Mac. The benefits of using this interface include your not having
to assign a unique ID number to each of your USB devices (this is done
for you by your computer), there is no need to terminate the last device
in the USB chain, each USB port can support up to 127 devices, and the
USB interface supports hot swapping (USB devices may be added and removed
while your computer is turned on).

You ask, “Is there a downside to using USB-based devices?” Of course
there is. While in theory USB devices should support hot swapping,
after connecting some USB devices to your Mac, they won’t work until you
restart your computer. Also, some USB hard drives can’t be used as a
startup volume, and some USB devices won’t work properly unless they are
connected directly to a USB port on your computer. (In theory, you
should be able to plug a USB device into a multi-port USB hub which
expands the number of USB ports that are available to you.) Perhaps the
biggest downside is speed or, more specifically, the lack thereof. The
current USB interface supports a maximum speed of 12 Mbps (megabits per
second), and this is only a small fraction of the speed that is supported
by both the SCSI and Firewire interfaces.

Also, each device that is on a USB chain shares the bandwidth (data
carrying capacity) for the port on the Mac that it’s connected to.
Consequently, when several USB devices are attempting to simultaneously
transfer data, they must share the already limited capacity of the USB
interface (this can dramatically slow down these devices and your Mac).
For this reason, in the real world, it’s not likely most users would want
to have many USB devices connected to these low-capacity USB ports.

Basically, most USB hardware like printers, scanners, and storage devices
are convenient to use, but they are relatively slow when compared to
their SCSI and Firewire counterparts.

To make things a little more interesting (maybe I should say confusing),
there is a USB 2.0 interface that may be appearing very soon, and it will
support a data transfer rate of up to 480 Mbps. However, even though in
theory it may be faster than Firewire, it has yet to prove itself, Apple
hasn’t yet agreed to support it, and there presently are virtually no
devices that work with it (some USB 2.0 compatible devices are scheduled
to ship this Spring).

SPECIAL NOTE: You should be careful when comparing performance
specifications between these interfaces. While both SCSI and Firewire
performance are often stated in MB/sec (megabytes per second), USB
performance is commonly stated in Mbps (megabits per second).

Since there are 8 bits (a bit is the smallest unit of information that
your computer works with) per byte, the USB rating of 12 Mbps translates
to only 1.5 MB/sec, so you can see USB performance is far below that of
either SCSI (up to 160 MB/sec) or Firewire (up to 50 MB/sec).

Also, while “port” and “bus” are often used interchangeably, the port is
the place on your computer where you connect a cable to the bus (the
“roadway” your data travels on in your computer). Bandwidth indicates
the information carrying capacity of this “roadway.” The interface (SCSI,
Firewire and USB) determines the “rules of the road” (the electronic
specifications that control how your data is handled).

THE Firewire INTERFACE

With this background information, you now have some perspective for
weighing the pros and cons of using Apple’s Firewire technology. The
pros are that Firewire is capable of supporting a data transfer rate of
50 MB/sec (more about this later), supports hot swapping of devices,
supports a chain of up to 63 devices per port, does not require you to
assign an ID to your Firewire devices, and Firewire devices do not need
to be terminated.

Another nice feature of Firewire (USB and SCSI don’t do this) is that it
reserves 3.5 MB/sec of bandwidth for many types of video equipment that’s
attached to your Mac. Since, like USB and SCSI, your Firewire devices
share the available bandwidth, this reserved bandwidth guarantees your
video performance won’t deteriorate if other devices are also running on
your Firewire chain—very cool!

You are probably saying to yourself, “Hmmm…seems like Firewire includes
the best features of both the SCSI and the USB interfaces.” In theory,
this appears to be the case; in practice, there are some caveats (myths)
to be aware of.

Firewire is a relatively new technology, and there are still some bugs to
be worked out (Apple is making good progress with this, though). For
example, as with USB, not all external Firewire drives are able to boot
your Mac, and not all external Firewire devices are fully plug and play.
The biggest myth is about speed; there is a huge gap between the
theoretical performance of most Firewire devices and the actual
performance you will experience when using these devices (SCSI devices
also will not always be able to perform at the theoretical maximum SCSI
limit).

Storage devices (i.e., various types of drives) often contain a “bridge”
chip, and the function of this chip is to provide communication between a
non-native Firewire device and your computer’s Firewire bus (port). This
chip is required because most storage devices are not yet native Firewire
devices, so without this “bridge” your computer wouldn’t be able to
communicate with these devices. The “weakest link” theory applies here:
the slowest point in the chain of communication between your computer and
your Firewire device determines the fastest speed at which this
communication can take place.

The bridge chips used in today’s Firewire storage devices usually support
a maximum data throughput in the range of 12-16 MB/sec. While this is
much faster than even the theoretical maximum the current USB interface
can handle, high speed SCSI devices are able to support a much higher
data throughput rate. However, a faster bridge chip capable of
supporting about 40 MB/sec is just becoming available, so it should soon
be possible for storage devices to realize much of the potential that is
offered by Firewire.

CONSUMER ALERT

While you see storage devices claiming they support Firewire’s 50 MB/sec
capability, you most likely will never approach this level of performance
with your system until the new bridge chips are in use. Further, most
consumer hard drives and other storage devices—regardless of the
interface being used—aren’t even capable of reading and writing data
at 50 MB/sec (probably 20-30 MB is a more realistic number for the better
hard drives), so I believe it’s misleading for companies not to
supply/advertise the “real” limitations/peformance capabilities of their
equipment (of course, this also applies to SCSI devices).

I expect that most of today’s consumer scanners, video equipment, etc.
also aren’t capable of generating data at a rate that takes full
advantage of Firewire’s potential, though it would be difficult to tell
this from their advertisements.

THE BOTTOM LINE

If and when the USB 2.0 interface becomes available for Mac users, it may
provide some serious competition for Firewire. (Apple is currently
working on a new Firewire spec that may boost Firewire’s performance to
800, 1600, and even 3200 Mbps.) At the moment, Firewire is dramatically
faster than USB and provides similar ease-of-use benefits.

If speed is your most important criteria when deciding whether to use
either SCSI or Firewire storage devices, some versions of SCSI
potentially provide real-world performance that exceeds that of most
Firewire storage devices, though at a cost of sacrificing the ease of use
that is provided by Firewire. Speaking of cost, SCSI devices also
frequently cost more than both USB and Firewire devices (I believe this
is at least partially due to the SCSI licensing fee).

Since SCSI has been around for much longer than Firewire, there still are
more SCSI-equipped than Firewire-equipped devices available, but the
availability of devices that include a Firewire port is growing rapidly.
Also, using the bridge chip enables companies to use Firewire technology
with inexpensive IDE drives, and this already has resulted in some very
attractive performance/price ratios for Firewire hard drives.

In my office, I use SCSI, USB, and Firewire devices, and my clear
favorite is Firewire! I have both a VST full-height 30 GB external hard
drive and a VST 10 GB Ultra Thin USB/FireWire combo drive. I use VST’s
excellent formatting software and have found these drives to provide a
wonderful combination of performance and ease of use, though there are
less expensive Firewire hard drives available from other vendors.

Unless you need the very highest performance that is available, I believe
that of the three interfaces, you are likely to find Firewire provides
the best combination of performance, ease of use, and price!

BTW, if you have a Mac (or Mac clone) that isn’t equipped with Firewire
but does include PCI (Peripheral Component Interconnect) slots, you can
install a relatively inexpensive Firewire PCI card that will add Firewire
ports to your Mac (be sure to also check on the minimum hardware and
system requirements for the card).

There are also some reasonably priced Firewire/USB combo PCI cards on the
market that you might want to consider (since most Macs have no more than
3 unused PCI slots, a combo PCI card includes two different types of
ports while using only one of these valuable slots). If you want to add
a SCSI port to your Mac, SCSI and SCSI combo cards are also available.

If you have a PowerBook that doesn’t include either a USB or Firewire
port, it may be equipped with a CardBus slot that will accept a Firewire
or USB card (if your PowerBook has this slot, your manual should include
information about it).

A FINAL NOTE

Please note that this article does not attempt to cover all aspects of
using Firewire, USB, and SCSI devices with your Mac. I hope you will
find it has clarified the major differences between the three interfaces
and has assisted you in deciding which technology best fits your needs!

© 2001 by Steve Becker. All rights reserved.

Steve Becker is a Mac consultant, author, and programmer.
Other articles written by Steve, as well as free demos of
his Mac shareware utilities, are available on Steve’s web site:
http ://www.kagi.com/MacEase