USB Frequently Asked Questions  
Hi-Speed USB The Universal Serial Bus (USB) specification is a widely used standard for connecting peripheral devices to your PC. USB makes connecting printers, digital cameras, scanners, digital joysticks, keyboards, mice, fast storage units and other PC peripherals and devices a true plug-and-play experience.
USB Common Questions
  1. What do USB ports and plugs look like?
  2. How many USB peripherals can I connect at once?
  3. What is the difference between USB 1.x and USB 2.0?
  4. Can I use game port, serial, parallel, or PS/2 devices on USB?
  5. How long can a USB cable be?
  6. How far away from my PC can I put a USB device?
  7. Can I connect two PCs with a USB cable?
  8. Can two PCs share the same USB printer or device?
  9. What are low/high power devices and bus/self powered hubs?

What do USB ports and plugs look like?
The USB cables most commonly used for connecting USB peripherals to your computer are A to B USB cables (pictured below) and A to mini B USB cables (for smaller devices such as digital cameras). Some equipment vendors also use non-standard B connectors for various special devices.
USB ports and plugs
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How many USB peripherals can I connect at once?
Theoretically, you can connect up to 127 individual devices to a USB bus. But since USB peripherals tend to reserve bandwidth, the practical limit is much less than the theoretical maximum. However, by using PCI-USB add-in cards, you can easily add independent USB buses to your PC to which even more peripherals can be connected.
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What is the speed difference between USB 1.x and USB 2.0?
USB 1.1 carries data at the rate of 12 megabits per second (12Mbps), which is sufficient for medium to low-speed peripherals such as mice, keyboards, digital joysticks and some printers. USB 2.0 is a new standard that was developed to support a new generation of hi-speed peripherals such as MPEG video-based products, digitizers, digital video camcorders and cameras. High Speed USB 2.0 products have a design data rate of 480Mbps. Full Speed USB devices run at 12Mbps. Low Speed USB devices run at 1.5Mbps.
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Can I use game port, serial, parallel, or PS/2 devices on USB?
Although you cannot plug these devices directly into USB, a variety of adapters exist that convert game port, serial, parallel, or PS/2 electrical signals into USB signals. These USB adapters allow you to attach non-USB devices to the USB port of your PC. This is particularly handy with newer laptops that do not include serial or parallel ports.
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How long can a USB cable be?
In practice, the USB specification limits the length of a cable between high/full speed devices to 5 meters (a little under 16 feet 5 inches). For a low speed device the limit is 3 meters (9 feet 10 inches).
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How far away from my PC can I put a USB device?
With a high/full speed device, you can connect the maximum of 5 USB hubs using 5m cables to get a 30m long cable. With a low speed device, you will be able to get a range up to 18m. You can also use 5m active extension cables (effectively a 1-port bus powered hub/repeater) to extend the distance of a USB cable. If you are connecting a high power device (and it draws its power from the usb bus) 10m or more away you should use USB hubs with external power supplies to avoid any potential power issues.
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Can I connect two PCs with a USB cable?
You need a specialized USB peripheral known as a USB bridge (sometimes called a USB Data Transfer cable or USB Network cable) to do this. Without a USB bridge, connecting a USB port of a PC directly to the USB port of another PC can damage the computers. Some cable vendors make A to A cables, but these cannot be used to connect two computers directly as this could possibly destroy one or both PCs or cause a fire hazard.
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Can two PCs share the same USB printer or device?
Yes, USB Switches enable you to have two computers sequentially share one of more USB devices such as printers and scanners.
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What are low/high power devices and bus/self powered hubs?
You may connect high power devices to self powered USB hubs only and low power devices to either bus or self powered hubs.
High power devices are devices that draw more than 100mA; low power devices are ones that draw 100mA or less. Examples of high power devices are bus-powered cameras and bus powered hubs. Low power devices are typically mice, keyboards, joysticks, and any devices that come with their own power supply (also known as self powered devices). Some general purpose hubs come with their own power supply and therefore are self powered. A subset of hubs called bus powered hubs gets power from the USB bus and therefore has the limitation of only supporting low power devices.
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