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USB FAQ | ||||||||||||||||
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| Frequently Asked Questions on USB | ||||||||||||||||
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Q1:
Will I
need
special
software
to run USB? A1: All Current operating systems support USB. Many devices get all of the software support they need from the OS, but there are some that will come with a floppy or CD with specialized drivers for that particular device. Don't worry about getting the right software because the OS's that support USB always know how to determine if anything else is needed and they will help you get the software loaded so the device works correctly. | ||||||||||||||||
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Q2:
Is USB
available
on mobile
computers
in
addition
to
desktops? A2: USB is currently a key feature on virtually every new notebook. One great advantage of USB is that it allows notebook users to easily share peripherals | ||||||||||||||||
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Q3:
What kinds
of USB
peripherals
can I
connect to
my PC? A3: USB carries data at the rate of 12 megabits per second, which is sufficient for "medium to low-speed peripherals". This is a broad category which includes telephones, digital cameras, modems, keyboards, mice, digital joysticks, some CD-ROM drives, tape and floppy drives, digital scanners and specialty printers. USB's data rate also accommodates new generation of peripherals such as MPEG-2 video-base products, data gloves and digitizers and soon an interface for Integrated Services Digital Network (ISDN) and digital PBXs. | ||||||||||||||||
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Q4:
Does USB
affect the
cost of
PCs and
peripheral
devices? A4: No. In most PCs, its a standard device interface fitted at no extra cost. By eliminating add-in cards and separate power supplies which you may need to add external devices, USB can help make PC peripheral devices more affordable. | ||||||||||||||||
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Q5:
How many
USB
peripherals
can I
connect at
once? A5: Technically, you can connect up to 127 individual USB peripherals at one time. Due to the fact that some devices reserve USB bandwidth, the practical maximum of devices is less than the theoretical maximum. However, PCI-USB add-in cards provide an independent USB bus to which even more peripherals can be connected | ||||||||||||||||
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Q6:
Where can
I learn
more
technical
details
about USB? A6: Technical presentations and other details are available from the USB Implementers Forum on the USB developers site, as well as directly from USB-IF member companies. | ||||||||||||||||
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Q7:
What does
it mean
when a
product is
USB
compliant
and why
should I
care? A7: Compliance means products that are called USB products are really built to match the description in the USB specification. There
are many
ways to
make sure
products
work
together
and many
ways to
see if
they match
the
specification,
but
compliance
testing is
one of the
most
useful. | ||||||||||||||||
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Q8:
How long
of a cable
can I use
to connect
my device? A8: In practice, the USB specification limits the length of a cable between full speed devices to 5 meters (a little under 16 feet 5 inches). For a low speed device the limit is 3 meters (9 feet 10 inches). | ||||||||||||||||
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Q9:
Why can't
I use a
cable
longer
than 3 or
5m? A9: USB's electrical design doesn't allow it. When USB was designed, a decision was made to handle the propagation of electromagnetic fields on USB data lines in a way that limited the maximum length of a USB cable to something in the range of 4m. This method has a number of advantages and, since USB is intended for a desktop environment, the range limitations were deemed acceptable. If you're familiar with transmission line theory and want more detail on this topic, take a look at the USB signals section of the developers FAQ. | ||||||||||||||||
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Q10:
How far
away from
a PC can I
put a USB
device? A10: With the maximum of 5 hubs connected with 5m cables and a 5m cable going to your full speed device, this will give you 30m of cable (see section 7.1.19 for details). With a low speed device, you will be able to get a range up to 27m, depending on how long the device's cable is. With a straightforward cable route, you will probably be able to reach out 25m or so from the PC. | ||||||||||||||||
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Q11:
How can I
connect
two PCs to
each other
with USB? A11: You need a specialized USB peripheral known as a USB bridge (sometimes called a USB to USB adapter) to do this. Anchor Chips and e-Tek labs, among others, make USB bridges. | ||||||||||||||||
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Q12:
Can we
make
direct
cable
connection
like a
null
modem? A12: Correct. In fact, if you try this with an illegal A to A USB cable, you'll short the two PCs' power supplies together, possibly destroying one or both machines or causing a fire hazard. Even there were no danger to the machines from the problem with two power supplies, there still wouldn't be any way to get the two PCs talking to each other, since USB doesn't support that particular kind of communication. A reasonably priced solution to handle this need is the USB bridge. | ||||||||||||||||
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Q13:
Why we
make A to
A cables?
What kinds
of cables
do I need
to connect
USB
devices
together? A13: The only kind of cables you'll ever need to connect normal USB products are A to B cables, A to mini B cables or mini A to mini B cables of various lengths. | ||||||||||||||||
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Q14:
What if I
want to
network a
whole
bunch of
PCs
together
with USB? A14: If you need to connect just a few machines, USB bridges and a hub will work. USB was not designed to be a LAN and there are certain safety hazards associated with trying to use USB with large numbers of PCs. There's also a large performance penalty compared to a real LAN. If you need a LAN, use a technology intended to be used as a LAN, such as Ethernet | ||||||||||||||||
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Q15:
Is there
any way I
can put a
USB device
on a
network,
like a
network
printer? A15: To do this, you'd require something like a USB to Ethernet bridge that is capable of acting as an USB host. Unfortunately, no one makes one of these at the moment. | ||||||||||||||||
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Q16:
How fast
is USB? A16: High speed USB products have a design data rate of 480 Mb/s. Full speed USB devices signal at 12Mb/s, while low speed devices use a 1.5Mb/s subchannel. | ||||||||||||||||
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Q17:
How we
compare
USB to
other
connections? A17: Here's a quick list of the maximum transfer rates for various connections in megabits (Mb) and megabytes (MB) per second: 1.
Serial
port:
115kbits/s
(.115Mbits/s) | ||||||||||||||||
| Note: The fastest connection commonly found on PCs is UltraIDE, which is used for hard drives and CD-ROMs | ||||||||||||||||
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Q18:
Will using
a USB to
serial or
parallel
adapter
free up
interrupts
on my
system? A18: Yes, but you'll have to manually disable the serial or parallel ports in your BIOS before something else can use the IRQ. | ||||||||||||||||
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Q19:
How does
USB
compare to
IEEE-1394? A19: While they look similar, they are intended to fulfill different market and cost needs. 1394 has the potential to move more data in a given amount of time, but is considerably more expensive than USB due to its more complex protocol and signaling rate. Applications that are best suited for 1394 are high quality consumer or professional video streams and other high bandwidth entertainment applications. | ||||||||||||||||
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