Tech Talk (04/08)
Understanding USB and Firewire
by Brian K. Lewis, Ph.D.*
Member of the Sarasota Personal Computer Users Group, Inc.
Both USB and Firewire connections have
changed since I last wrote about them in the
Monitor (Dec 2005). There are also more changes on the horizon for both
of them. So it is time to review again what they are, how they work and
what they can be used to accomplish.
USB (universal serial bus) is now the most common
method of connecting computers to many peripheral devices such as
printers, scanners, video cameras, external drives, mice, keyboards,
etc. USB started out working at a maximum speed of 12 megabits per
second (Mb/s) and was revised in version 2.0 to a maximum of 480 Mb/s.
In reality, USB never achieves these maximum values. In real time use,
the throughput is closer to 50% of the rated speed or less. To get a
little better understanding of these speeds you need to understand what
a megabit means. In computer terminology a bit stands for a zero or one
in the binary language of computing. It takes 8 bits to equal one byte.
For example, a computer will convert the decimal number five to a binary
number containing 8 bits. So a mega bit is a million bits and a megabyte
is a million bytes. Essentially, 480 megabits per second is the
equivalent of 60 megabytes per second. A USB connection doesn’t sound
nearly as fast when expressed this way, does it?
The cable used for USB connections contains four
wires, two for data communication and two for power. The transmitted
power is 5 volts which is provided by the computer bus. The maximum
length of a USB cable is 5 meters (16.4 ft). This allows for the maximum
turn-around time of 1500 nanoseconds for the USB signal. This distance
can be extended by means of "repeaters" such as powered hubs or active
cables.
In most low-powered uses the devices connected to the
USB port use the power from the computer bus. However, each computer
port is limited to a maximum of 500 mA (milli-amperes) of current. If
the device requires more power, then it must have its own power
connection. Frequently, users find that their computer doesn’t have
enough USB ports for all the connections they wish to make. In theory,
you can connect up to 127 devices through a USB chain. This chain is a
star formation. That means you can connect a hub directly to the
computer port. The hub has 4 to 7 ports. You can then connect another
hub to one port and USB devices to the other ports. Here is where some
users run into trouble. If the hub is powered by the computer, then each
port on the hub can only provide a maximum of 100 mA. Any device which
requires more power and is not self-powered will not work. The solution
to this is to use a powered hub. In other words, a hub that can be
connected to an electrical outlet. Powered hubs can provide the full 500
mA to each port. There is one more problem with hubs. If you plug in a
low-speed device (USB spec. 1.1), then all the ports on that hub will
revert to low speed mode. Any hi-speed device (USB spec. 2.0) plugged
into the same hub, will not function at the higher speed. In some cases,
the device will not work at all. For this reason, if you need a hub in
order to obtain more USB ports, you should purchase a powered USB 2.0
hub that requires its own electrical connection.
When you plug in a USB device to either a hub or a
computer port, there is an initial identification process referred to as
the "configuration" step that occurs. During this configuration process
the device can not draw more than 100 mA. If it does, the process will
fail and it will appear that the device failed to work. The
configuration process identifies the device, its drivers, and its power
requirements. Only after this process is complete will the device be
able to draw more than 100 mA current. Since this process is not
instantaneous, some time must be allowed by the user before attempting
to use the device. In addition, this configuration may not occur if the
device is plugged into the computer before the computer is turned on and
booted. In these cases, it may be better to connect the USB device after
the computer is fully operational. However, most of the time, the
computer will identify and configure the device during the bootup
process.
There is a new USB specification in the works, USB
3.0. The initial information on this specification includes a speed of 4
Gb/sec (gigabits/sec) or 300 MB/sec (megabytes/sec). One of the prime
movers behind this new specification is Intel. The need for increased
speed is related to the increases in the speed capability of USB
peripherals such as SATA external drives. Another aim is to keep up with
the speeds of flash chips in USB solid state drives (thumb drives, flash
drives, etc.). In addition, Intel hopes to compete with Firewire in the
world of audio and video, such as camcorders and external DVD drives,
where Firewire has been preeminent. The complete specification is
expected to be published later this year with USB 3.0 devices appearing
early in 2009.
Firewire is the name that Apple gives to its version
of the IEEE 1394 communications specification. However, the term really
has been become the generic term for this type of connection rather than
referring to IEEE 1394. Sony refers to their use as i.Link and Panasonic
has its own name of DV. Firewire is a form of serial communication
between devices like USB. Both Firewire and USB are capable of allowing
devices to be "hot-plugged". That simply means that they can be added to
or removed from the computer without shutting down the computer.
But there are many differences between the two
systems. The speed of communication is one of the major differences.
Firewire started at a speed of 100 Mb/s and was later increased to 400
Mb/s and then 800 Mb/s with a change in the cabling. Due to its design
Firewire can come much closer to its maxi mum speed in real time
conditions. So it is significantly faster than USB 2.0.
Another difference is that Firewire devices do not
use the star or hub configuration. They are connected in a "daisy chain"
and any one device can communicate directly with another device. For
example, a camcorder or scanner can communicate directly with a printer
without having to go through the computer. Firewire can connect up to 63
devices and the 800 Mb/s version can handle cables up to 100 meters in
length between each device. That means that if you have 5 devices in
your chain, the distance from one end to the other can be 400 meters.
In addition, power transmission with Firewire is much
higher than with USB. The cables carry up to 30 volts and 1.5 amps. In
this case, more devices can draw their power directly from the Firewire
bus. One other difference is that of cost. Firewire cables have 6 to 9
connectors, depending on the version. The controllers for Firewire are
more complex. In addition, Apple and other patent holders demand a
royalty payment from manufacturers of Firewire devices. All of this
increases the overall cost of Firewire compared to USB. This partly
explains the success of USB compared to Firewire.
Firewire is not standing still in its development.
Just like USB it has a faster version in the works. A new specification
has been published for a 3.2 Gb/sec version. It is backwards compatible
with the Firewire 800. Whether it will be compatible with Firewire 400
is not clear to me. Firewire 400 devices can be run in a Firewire 800
chain only with the help of special adapters for the cable connections.
I would expect devices using this new specification to show up later
this year.
For most uses, other than really high speed devices,
USB is more than satisfactory. When you become involved with digital
camcorders, external Blu-ray DVD drives, or external SATA hard drives,
then you need to consider adding Firewire capability to your computer.
Whichever you choose to use, they both have a significant advantage over
the older serial/parallel port systems.
*Dr. Lewis is a former university and medical school professor of physiology. He has been working with personal computers for over thirty years, developing software and assembling systems. He can be reached at bwsail at yahoo.com.
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Copyright 2008. This article is from the
April 2008 issue of the
Sarasota PC Monitor, the official monthly publication of the Sarasota
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