Specifications

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Communication Overview
23
COM
The Series A connector pictured (see bottom left) is for use with
high speed (12 Mbps) devices, and can be up to 5 meters long.
The more common of the two, it consists of one pair 20-28 AWG
wire for power (V
BUS
is typically +5V at the source) and one 28
AWG twisted wire pair for data. The connector has a shielded
housing, making it STP compliant. (See the drawing of a USB
cable below.)
Power Management
One special feature of USB systems is that they can directly supply
power to the peripherals and the hubs attached to them. It can
also regulate power usage for peripherals that use independent
power sources. USB devices are classified based on the amount
of power they supply or require. Low Bus Power devices take all
their power from the bus, but no more than 100mA at a time.
High bus-powered devices also take all their power from the bus,
but can draw up to 500mA at a time. Self-powered devices use
an external power supply, but can draw up to 1mA from the host
if necessary--such as in the case of a power failure.
Hubs can also be low, high or self powered. Power flows
downstream in a USB system, which means that a self-powered
hub can be used to power high- and low-powered peripheral
devices located further down in the network. This power
arrangement has both advantages and disadvantages. For desktop
systems where power is not a problem, it is extremely convenient
not to have to use a separate outlet for each peripheral connected
to the PC. In notebooks where battery longevity is often a
problem, it might be more advantageous to use peripheral devices
that have their own power source.
Looking Ahead
USB Specification 1.1 was designed for low to medium speed
applications running at less then 12 Mbits/sec. As such it is not
suited for high-end data transfer such as high-speed back-ups
to hard disks or CDs , high resolution color printing and interactive
gaming. The recently released USB Specification 2.0 aims to
upgrade the bus for high performance applications. The main
difference between Specification 1.1 and 2.0 is that the latter
provides for data transfer rates up to 480 kbits/sec.
(USB Cable)
USB 2.0 is fully backward compatible with all older USB devices.
It merely adds another device class--"high speed device." The
USB host controller determines the type of devices attached to it,
and then treats them accordingly. In fact, a high-speed USB hub
can be used for both high, full (12Mbps) and low (1.5Mbps)
speed devices at the same time.
While still relatively new, many manufactures are starting to release
2.0 peripherals. Quatech's serial USB adapters will remain USB
1.1 devices, as even the fastest serial communication is limited
to 10Mbps--well within the range of a full speed device.
V
BUS
D+
D-
GND
USB for Data Communication
For low to medium speed data communication applications USB
Specification 1.1 provides a clear usability advantage older
bus types. USB peripherals are both Plug and Play and
Hot Swappable devices. Further, USB is flexible enough
to incorporate up to 127 individual devices into a single
system using only one interface. And, unlike PCMCIA
cards, where the board itself is subject to considerable
wear from multiple insertions and extractions, USB devices
use a connector cable which can be inserted and removed multiple
times without consequence. Because of USB's structure, it can
potentially reduce system downtime considerably.
As a bus option designed for both desktop and portable use,
USB can bridge the gap between desktop and portable peripherals,
provided the new peripherals are designed in small enough form
to be practical for portable systems, and provided they do not
draw too heavily from a laptop's limited battery power.
USB 2.0 with its considerably higher speeds will rival both board-
level interfaces such as PCI and other interfaces such as IEEE
1394 (see next page). In fact, some computer companies are
pushing for a PC standard that will no longer supply slots for
plug-in boards, and will rely completely on USB and IEEE 1394
type devices. With the knowledge that IEEE 1394 has become
the standard for video applications, USB 2.0 has been positioned
as a more general purpose, and low-cost, solution.
USB's major drawback, one it shares with IEEE 1394, is its inability
to implement peripherals designed for older protocols. As USB
popularity increases, it is becoming more likely that a USB device
exists for any given application. However, software applications
written for non-USB peripherals cannot be implemented using
USB because of the difference in communication protocols.
Quatech has solved this problem with our FreedomUSB serial
adapters (see pages 86-93). With Quatech's FreedomUSB Series
you can take full advantage of USB benefits while continuing to
use your current serial peripherals in your existing applications.