January 2003: Plugging into USB

January 2003: Plugging into USB
January 2003 Vol.5 No.1
Inside this issue:
• About "USB 2.0 Cables"
• Maximum USB Cable Length
• USB Logos
• Lava USB Products
• Distributor Profile
Lava I/O News
Plugging into USB
On USB Cabling
Confusion has been occurring lately about
types and qualities of USB cabling. A number
of manufacturers are selling "USB 2.0 Cables,"
and customers have been wondering how
those differ from regular USB 1.1 cables.
The confusion is understandable, as the plugs
and receptacles for both USB 1.1 and USB 2.0
are the same. So what's the difference? Here's
the dirt: there is no difference. If a cable meets
the specification for 12 Mbps USB,it meets the
specification for 480 Mbps USB. A cable called
a "USB 2.0 Cable" should be capable of
working with both USB 1.1 and USB 2.0 data
and peripherals, at the speed of the interface
being used. Similarly, a cable called a "USB
Cable" or a "USB 1.1 Cable" should also be
capable of operating in precisely the same
way, with the same data transfer rates and
reliability.
Up to Spec
A look at the USB specification shows why.The
Universal Serial Bus Specification Revision 2
defines all speeds of USB: Low-Speed, FullSpeed,and High-Speed (1.5,12,and 480 Mbps
respectively). It includes descriptions of three
basic cable assemblies: the standard
detachable cable,the high-/full-speed captive
cable, and the low-speed captive cable. The
captive cables are just as their names suggest:
captive to a device; that is, not removable. A
good example of a low-speed captive cable is
that used on a USB mouse.
All USB cables have five wires: two power
conductors, two signal conductors, and a 28
AWG drain wire. Beyond these similarities,
important distinctions exist between lowand high-/full-speed cables. The low-speed
cable is designed for low-speed signaling
only,so twisted-pair conductors and a braided
outer shield are recommended but not
required by the specification. Consequently,
low-speed cable can be thinner than cable for
high-/full-speed signaling. The diagrams on
the next page show these differences.
In terms of connectors, low-speed cable has a
Series "A" plug on one end, and the other end
is either hard-wired to the device or uses a
vendor-specific detachable connector. Apart
from vendor-specific designs, detachable
versions of the low-speed cable are
prohibited by the specification.
The standard detachable cable and the high/full-speed captive cable are the same as each
other in their cabling and materials
requirements, except that the detachable
cable is terminated on one end by a Series "A"
plug and on the other end by a Series "B" plug,
while a high-/full-speed captive cable has a
Series "A" plug on one end and is hard-wired
or uses a vendor-specific detachable plug on
the other.
Specific details on the gauges and materials of
the individual wires of a USB cable are outside
the scope of this newsletter, but it boils down
to this: the USB 2.0 specification that governs
USB today makes no distinction between the
cables used for High-Speed USB and the
cables used for Full-Speed USB. So, when it
comes down to it, there is only one design of
detachable cable, aside from vendor-specific
designs. If it has Series "A" and Series "B"
connectors,it is a high-/full-speed cable.
How long is too long?
What is the maximum length permitted for a
USB cable? The answer, my friend, is that the
USB specification defines maximum cable
length in terms of what is required for a cable
to properly handle a USB signal, rather than
providing a simple linear measurement.When
we are told that the maximum cable length for
a USB cable is 15 feet, what we are getting is a
digested version of the truth. "Fifteen feet"
appears nowhere in the specification.
What the specification does say for detachable
cables is that the maximum allowable cable
length is determined by limits on signal pair
attenuation (how much the signal pair loses
signal over distance), voltage drop across the
ground lead (to a maximum of 125mV from
upstream to downstream), and propagation
delay (how long a signal takes to travel a given
distance of cable).Given that the first two can
Series "A" Connectors
Series "B" Connectors
Series "A" plugs are
oriented upstream
(towards the USB
host or hub).
Series "B" plugs are
oriented downstream
(towards the USB device).
Series "A" receptacles
provide
downstream output
(from the USB
host or hub).
be controlled in significant measure by the
cable, the really limiting factor is propagation
delay. The specification allows a maximum of
70 ns of propagation delay through the
upstream cable and hub electronics. Of this,
the maximum one-way cable delay allowed is
26 ns. Given that the specification states also
that cabling can have no more than 5.2 ns of
delay per meter,we can see that 26/5.2 gives 5
meters as the maximum cable length–or 16.4
feet approximately.
Twisted
Data Pair
Series "B" receptacles
provide
upstream output
(towards the USB
host or hub).
What's not allowed in USB
cabling?
Generally speaking, as far as USB cables are
concerned, if you have to ask whether
something is allowed, the answer is no. Even
so, the specification does enumerate a few
explicitly prohibited cable designs.
First, extension cable assemblies are not
allowed. These are cables terminated with a
Series "A" plug on one end and a Series "A"
receptacle on the other,or a Series "B" plug on
one end and a Series "B" receptacle on the
other.These designs allow multiple segments
Black (Ground)
White (D-)
of USB cabling to connect, and possibly
exceed the maximum permitted cable length.
While they do exist, and are perhaps even
useful at times, extension cables are outside
the specification.
Second, cables with Series "A" plugs on both
ends or with Series "B" receptacles on both
ends are not permitted. These cables would
allow two downstream ports to be directly
connected, which would violate the
hierarchical USB topology. Unlike FireWire,
USB demands that peripherals be connected
to hubs,not to other peripherals.
Third, standard detachable cables are
prohibited for low-speed devices.You will not
(or should not) see a USB mouse with a cable
connector on the mouse, rather than a hardwired cable.Because the standard detachable
cable is rated for full and high speeds, using it
exceeds the capacitive load of low speed USB.
Green (D+)
On USB Logos
Red (VBUS)
65% Tinned Copper
Braided Shield
28 AWG STC
Drain Wire
Aluminum Metalized
Polyester Shield
Polyvinyl Chloride
(PVC) Jacket
High-/Full-Speed Cable
Low-Speed Cable
2
Three USB logos currently exist: the
traditional, original USB logo, the "Certified
USB" logo, and the "Certified Hi-Speed USB"
logo. The success of USB caused the USB
Implementers Forum (USB-IF) to realize the
value of their trademarked logo, and inspired
them to control its use. To do this best, they
developed two new logos, as the original USB
logo was effectively in free general use. The
"Certified USB" logo is licensed for use by
manufacturers whose product signals at 12 ,
Mbps or 1.5 Mbps, has been submitted to and passed the USB-IF Test Procedure for Basic-Speed
products, and has been posted on the USB-IF Integrators List.The USB-IF Hi-Speed logo similarly
applies to products signaling at 480 Mbps.The USB-IF states that the granting of license rights to
the logos does not constitute a "certification" of the product by the USB-IF, despite the word
"CERTIFIED" on the logos.
In addition to the USB logos,the specification includes a USB icon.This is the design required to be
embossed into the plugs of a USB cable, as well as located adjacent to each USB receptacle.
USB Logo Spotting Guide
The original
USB logo.
The "Certified USB"
logo for low- and
full-speed devices.
The "Hi-Speed Certified
USB" logo for high-speed
devices only.
The USB icon for
identifying all
USB connectors.
Lava USB Products
In the USB world, adding ports with host
adapters makes especially good sense.
Increasing the number of USB ports in a
system by employing a host adapter adds
full bandwidth on each new port.
USB 2.0 Host Adapter
Lava's USB 2.0 Host Adapter adds two USB
2.0 ports, each with up to 480 Mbps of data
throughput. USB 2.0 is currently natively
supported in Windows 2000 and ME, and
will connect both USB 2.0 peripherals as
well as USB 1.1 devices.
The workhorse Lava USB 1.1 Host Adapter
can transfer data at up to 12 Mbps.This host
adapter is supported in Windows 98SE, ME,
2000,and XP;Mac 8.6+;and Linux 2.4+.
USB 1.1 Host Adapter
Kazan USB 2.0 Drive Enclosure
Lava's Kazan – the first USB 2.0-to-IDE drive
enclosure on the market – makes great use
of the speed and convenience of USB 2.0 by
combining USB 2.0-to-IDE bridging
electronics, an external power supply, all
cables, and a sturdy and compact
enclosure. Pop in any IDE drive and you
have a high-speed, hot pluggable external
drive! It's perfect for taking work between
home and office, or for quickly and easily
backing up data.
The Lava SPH-USB 1.1 Hub gives real
versatility to both notebooks and PCs by
adding three powered downstream USB
ports, a bi-directional parallel port, and a
16550 UART serial port.It is an effective way
to add these ports to systems lacking PCIbus expansion capability or free PCI slots.
SPH-USB 1.1 Hub
3
Profile
Cables To Go delivers the highest quality
connectivity products to resellers via
direct sales and through key distributors
such as Tech Data and Ingram Micro.
Their innovative and timely solutions are
the result of more than 15 years of
industry experience. Whether it’s a
standard product or a custom cable,
they are an expert source for resellers'
connectivity needs.
C a b l e s To G o u s e s t h e f i n e s t
components and the latest construction
technologies to maximize cable
performance and to ensure compliance
to industry specifications for each cable
and its designated application.
From PC Cables and Adapters to
Networking and Data Sharing products,
Cables To Go delivers the best
products–including Lava boards–at an
always competitive price. In addition,
they keep packaging simple to let
resellers customize the final sale to their
customers to ensure a superior
connection every time.
When a reseller is looking for a complete
connectivity solution, Cables To Go can
also help.Their 15 years of experience as
a custom cable manufacturer enables
them to provide built-to-specification
connectivity solutions when and where
you need them–all at a competitive
price. Their consultative approach to
sales and engineering helps resellers to
determine application requirements,
and enables Cables To Go to rapidly
accommodate customers’ evolving
needs.
CablesTo Go
1501Webster Street
Dayton,OH 45404
TEL: 937.224.8646
FAX: 937.496.2666
TOLL FREE FAX: 800.331.2841
www.cablestogo.com
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