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USB Cable Overview:

usb cable

The Universal Serial Bus or USB cable was developed around the idea that users should be able to run multiple peripherals on their computers without the hassle of physically installing boards, manually allocating system resources, individually configuring devices and powering the computer up and down every time equipment needs change. With USB cable, up to 127 individual peripheral devices can be connected to a host computer using a single interface and a system of USB hubs. (See below for a diagram of a typical USB system.) Attaching a USB peripheral to your computer is as easy as plugging headphones into your Walkman. USB devices are automatically recognized and configured. They can draw power directly from the system, from an attached self-powered hub or be connected to their own power supply.

USB Cable Features

USB provides two-way communication between the PC and peripheral devices, making it ideal for many I/O applications. Multiple devices can connect to a system using a series of USB hubs and repeaters. A single USB interface is attached to the motherboard. A Root Hub with up to seven additional ports can be integrated into the main interface, or it can be externally connected with a cable. Each of the seven hubs on the Root Hub can in turn be connected to seven hubs, etc. to a maximum of seven tiers and 127 ports. A unique feature of USB is that a peripheral device can have a hub built into it. This type of peripheral, called "compound devices," are comprised of a function device and one or more hubs. For example, a USB keyboard can contain an additional USB port for a USB mouse.

USB is generally described as having a tiered star topology, however each device communicates with the host as if it had its own connection. This means that communication from the host centers around a set of hubs/devices, each of which in-turn serves as the center for another set of hubs/devices, etc. However, the hubs are transparent to the software and the devices are addressed individually. Cables are used to create point-to-point connections between devices and USB ports, or to connect one USB hub to another. The maximum cable length is five meters long. However, a repeater hub may be used to extend the distance between the peripheral and the host. There are also special USB repeaters that can be used to extend the connection even further.

USB Cables and Ports

The Root USB Hub is connected directly to the USB Host, and from there, everything is done with cables. Two types of USB cables can be used with USB devices: Series A and Series B. Series B cables are limited to 3 meters in length and are for use with low-speed (1.5 Mbps) USB peripherals such as keyboards and mice. The UTP cable has a pair of 28 AWG wire stranded copper for data and one pair 20-28 AWG for power.

The Series A connector 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 (VBUS 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.

USB Cable 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.

USB 2.0

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 Mbits/sec.

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.

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 cable 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 rivals both board-level interfaces such as PCI and other interfaces such as Firewire. 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 Firewire type devices.

USB's major drawback 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. 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.