Article, March 2006
A USB Primer
by Brian K. Lewis, Ph.D.
Sarasota PCUG, Florida
By now most computer users are familiar with the term USB or Universal Serial Bus. This connection port on your computer is designed to replace the older serial, parallel and PS2 ports. Probably within a year you won't find any new computers with these older ports. They will have only USB. There are some things you might find useful about USB ports and hubs which could reduce or eliminate problems in dealing with them. USB ports have a number of advantages over the old system of parallel/serial ports. They do not require I/O memory space or individual IRQ lines. Anyone who has had to work with older computers and operating systems will remember the problems of trying to prevent IRQ conflicts when connecting external devices such as scanners or modems. How many times did the sound card manage to steal IRQ's that you had to have for another device? USB also allows for automatic device configuration and hot-plug capability. The hot-plug or hot-swap function means that you don't have to power down the computer and go through a restart when you want to connect a new device. In instead you simply connect or disconnect the USB cable. The computer will recognize the device and connect to the proper driver. That is assuming this isn't the first time you have used the device and that the driver has already been installed. You commonly have to install drivers for external hard drives, printers, scanners, card readers, etc. You generally don't have to install drivers for mice and keyboards that connect to the USB ports.
Next, consider that USB operates at three possible speeds: low speed or 1.4 megabits/second, full speed or 12 megabits/second, and high speed or 480 megabits/second (mbps). Low speed and high speed can be used with either USB 1.x or USB 2.0 hosts. The high speed can be used only with USB 2.0. The host is the computer that provides the USB connections. For USB 2.0 operation the host computers “root port hubs” must support USB 2.0. That means the computer must have USB 2.0 drivers that are supported by the operating system. Windows XP (service pack 1.0) and Windows 2000 both support USB 2.0. The root port hubs are the USB connectors on your computer and are usually connected to the motherboard. You can also identify them in the Device Manager where they will show under the USB Host Controller.
Although the USB specifications indicate you can daisy-chain up to 127 devices from one port, this is not likely to happen because of the power drop that occurs over long connections. You can get external hubs that allow you to connect 4 – 7 devices to a single root port hub. The external hubs are repeaters that relay transaction information from the computer to a device connected to the hub's port. There is a catch to this as well. Some USB 2.0 hubs will decrease their maximum output if you have a USB 1.x device attached to a port. This means that any USB 2.0 device attached to the hub may not function or will function at a lower speed. I learned this the hard way with an external USB 2.0 hard drive. I thought the problem was the drive, but it was the hub. I had a USB 1.0 card reader attached to the hub and as a result the current output to each port was reduced to the point that it was not sufficient to run the drive. This occurred even though the hub and the drive had external power supplies.
An ample power supply is necessary to operate USB devices. Root hub ports can provide 5 volts and up to 500 milli-Amps (mA) of current. The USB power specifications state the USB ports should provide between 100 mA and 500 mA. Devices connected directly to the computer are able to obtain the maximum current. So let's take a situation where we have a hub connected to the computer's root port hub. The hub is receiving 500 mA of current at about 5 volts. If only the current received through the connecting cable powers the hub, the output from each of its four hubs will be only 100 mA. This is referred to as a “bus-powered” hub because it receives its current solely from the USB bus in the host computer. However, if the hub has its own power supply and is receiving at least 1.6 Amps from this supply, then it can provide a 500 mA output at each port. In this case, the hub should be able to support high-speed USB 2.0 devices.
On some computers you will find an icon in the system tray that is a “hardware disconnect”. In this instance, you need to double click the icon and wait for it to permit you to disconnect the hardware device. This icon does not appear with all USB devices. Since installing SP2, I haven't seen this icon in my system tray. There are some reports of computers with SATA drives showing this icon. Since the SATA drive is supposed to be hot-swappable like USB devices, I'm not surprised that the icon shows up. However, with my Seagate SATA drive, I still don't see this icon. That may be because my motherboard doesn't support the hot-swap function even though it supports SATA drives.
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. Most of the time, the computer will identify and configure the device during the bootup.
Microsoft has a knowledge base article on trouble-shooting USB problems (#310575). Typically the problems relate to drivers or power problems. However, they also point out that high-speed devices should be connected with high-speed cables only. The low speed cables may distort the signal as a result of their reduced amount of shielding. Another really complete source of trouble-shooting information can be found at: http://www.usbman.com/winxpusbguide.htm. This has references for Windows ME as well as XP and Windows 2000. However, anyone who hasn't dumped Windows ME should seriously consider doing so as soon as possible. WinXP is such a tremendous improvement over ME and will really make your computing experience much more enjoyable. My computer hasn't locked up or crashed in more than six months, probably longer. I really don't keep track anymore.
So what devices are typically high speed and require USB 2.0? Let's start with external USB hard drives. Seagate has a very interesting technical paper on external hard drives. In this paper they state that most 2 ˝” external drives require 1000 – 1100 mA during the start-up cycle and then can function at the 500 mA maximum current available from the USB port. They also state that most USB ports can support up to 700 mA on a continuous basis. This is something I have not found elsewhere. Consequently, my recommendation is that when looking for an external drive, pick one with its own power supply. I wouldn't want to depend on one that drew all its power from the USB bus.
Other high-speed devices include laser printers, scanners, and multi-function printers. All of these should have their own power supply. Other devices that operate at full-speed and don't need external power are blue-tooth adapters and card readers. This is only a partial list of what is available in USB devices. Just remember when you are looking at them that the term full-speed does not 480 mbps, instead it is the slower 12 mbps.
Dr. Lewis is a former university & medical school professor. He has been working with personal computers for more than thirty years. He can be reached via e-mail: email@example.com.
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