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Frequently Asked Questions about USB 

Last Updated: August 2000

Questions:

What is USB? What is USB good for? What isn't USB good for? What is "hot-swapping"? What is "plug and play?" Hubs? Huh? Self-powered vs. powered hubs? How fast is USB? Can my computer use USB devices? How long can cables be? Type A or B / upstream or downstream? How do I daisy chain my USB devices? Where can I go for more information?

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(1)	What is USB? USB is the simple way to connect peripherals to your computer. It can be used to attach a wide variety of devices like scanners, cameras, keyboards, speakers - almost anything to your computer.

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(2)

What is USB good for? USB for BLE BORDER=0 CELLSPACING=0 CELLPABLE BORDER=0 CELLSPACING=0 CELLPADDING=0>

(7)

Self-powered vs. powered hubs? One important feature of USB is that the cables distribute power as well as data. This means that devices that use modest amounts of power don't need separate power supplies. So few big power cubes at your wall outlet and fewer cords running across your desk. Still, the amount of power distributed over USB is limited. A unpowered (or self-powered) hub uses some of the power coming to it for it's own operation, and passes the remainder along to devices plugged into it. This is OK for small hubs with low power devices plugged into it. A good example is this is a keyboard with an powered hub built-in. This has plenty of power for plugging in a mouse or track pad. Powered hubs have their own power supplies and can supply full power to all the devices that can physically plugged into them. Of course, you do have an extra wire and power block plugged into the wall. Some hubs can operate either way. If they don't have their power supply plugged-in, they operate as an unpowered hub - with a limited ability to power additional USB devices. When you plug-in their power supplied, they function as powered hubs.

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(8)

How fast is USB? USB has a "low speed" mode operating at 1.5 Mbps and a "high speed" mode operating up to 12 Mbps. Low speed mode is often used by slower devices such as mice, keyboards, and joysticks. High speed mode is required by faster devices like scanners and printers. Today (1998) this speed range places USB firmly in the midrange. SCSI and Firewire are faster, serial and ADB are slower. A few years from now USB will undoubtedly be viewed as low speed... Most computer with two USB ports place both ports on the same USB bus. This means that the bandwidth is shared between them. So, if you have USB speakers on one port, and a printer on the other, the printer will run slower when sound data is being sent to your speakers. New computers, such as Apple's G4, are using separate USB buses on each USB port. This means that you can use the full 12 Mbps bandwidth on each port.

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(9)

Can my computer use USB devices? This depends on two things: hardware and software. Hardware-wise, you need a USB port in your computer. This can either be built in (most PC's build in 1999 and Apple's iMac) or from a plug in card. A number of vendors sell PCI and CardBus USB adapters which can add USB capabilities to your older computer. Software-wise for Wintel computers, you need either Windows 98, Windows 95 OSR 2.1 (although Windows 98 has better USB support than Windows 95), Windows 2000. For Apple Macintosh computers you need MacOS 8.1 or later.

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(10)	How long can cables be? Five meters is the maximum cable length allowed by USB. You can achieve longer cable runs by inserting a hub every five meters. Companies are also introducing cables with repeaters built into them to allow longer cable runs.

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(11)

Type A or B / upstream or downstream? There are two main types of USB connectors you will encounter. Type A plugs are the USB output port of a host system or hub. These are also know as upstream ports. The USB plug in your computer is a Type A plug. Your typical USB device with a single cable coming out of it plugs into a hub or host systems Type A port. Type B plugs are the USB input ports leading into hubs. These are also know as downstream ports. They always connect to a Type A, upstream port, at the other end of the cable.

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(12)

How do I daisy chain my USB devices? USB devices are not daisy chained, rather they use hubs to connect more devices. If you have a single USB port on your computer and need to use three USB devices, you need to first plug a hub into the computer. The hub will then have four, or seven (or however many) ports available for you to plug in additional devices. Hubs can be daisy chained, to provide even more USB ports.

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(12)	Where can I go for more information? A comprehensive FAQ site about USB devices can be found at: http://www.usbfaq.org.

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How long can cables be?

Five meters is the maximum cable length allowed by USB. You can achieve longer cable runs by inserting a hub every five meters. Companies are also introducing cables with repeaters built into them to allow longer cable runs.

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©2000 allUSB.