USB Info: Frequently Asked Questions

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Q1:How fast is USB?
A1: High speed USB 2.0 products have a design data rate of 480 Mb/s. 
       Full speed USB 1.1 devices signal at 12Mb/s.
       Low speed devices use a 1.5Mb/s subchannel.

Q2:How does this compare to other connections used with PCs and workstations?
A2: Here's a quick list of the maximum transfer rates for various connections in megabits (Mb) and megabytes (MB) per second:

serial port: 115kbits/s (.115Mbits/s)
standard parallel port:  115kBYTES/s (.115MBYTES/s)
Original USB: 12Mbits/s (1.5MBYTES/s)
ECP/EPP parallel port: 3MBYTES/s
IDE: 3.3-16.7MBYTES/s
SCSI-2 (Fast SCSI, Fast Narrow SCSI): 10MBYTES/s
Fast Wide SCSI (Wide SCSI): 20MBYTES/s
Ultra SCSI (SCSI-3, Fast-20, Ultra Narrow): 20MBYTES/s
UltraIDE: 33MBYTES/s
Wide Ultra SCSI (Fast Wide 20): 40MBYTES/s
Ultra2 SCSI:  40MBYTES/s
IEEE-1394: 100-400Mbits/s (12.5--50MBYTES/s)
Hi-Speed USB 2.0: 480Mbits/s
Wide Ultra2 SCSI: 80MBYTES/s
Ultra3 SCSI: 80MBYTES/s
Wide Ultra3 SCSI: 160MBYTES/s
FC-AL Fiber Channel: 100-400MBYTES/s

    The fastest connection commonly found on PCs is UltraIDE, which is used for hard drives and CD-ROMs

Q3: So which is faster, a USB peripheral or a serial/parallel/SCSI/1394/etc. one?
A3: This depends on what the peripheral is and how it's made. For example, a USB keyboard won't let you type any faster than a normal PS/2 keyboard because your typing speed is limited by how fast your fingers can hit the keys, not by the speed of the keyboard's connection to the PC. Similarly, devices like printers and scanners may be able to run faster on USB than they can with a standard parallel connection, but only if the print head or scan engine can keep up with USB's higher data rate, which is not necessarily the case. On the other hand, USB makes possible peripherals that were impractical with earlier connections, such as video cameras and ADSL and satellite modems.

Q4: Can I use game port, serial, parallel, or PS/2 devices on USB?
A4: These devices use different electrical signals than the ones used for USB, so you can't plug them into USB directly. However, a variety of manufacturers make USB devices that will convert game port, serial, parallel, or PS/2 devices into USB signals. With these, you can actually attach many more non-USB devices to your PC than you could without USB. Note that some devices support multiple connections to the PC, and therefore come with special adapters that let them connect to USB or another port. These adapters don't perform any signal translation, so nothing will happen if you take one of these adapters and put it on another device that doesn't support USB.

Q5: Will using a USB to serial or parallel adapter free up interrupts on my system?
A5: Yes, but you'll probably have to manually disable the serial or parallel ports in your BIOS before something else can use the IRQ.

Q6: What about EIDE, SCSI and network adapters?
A6: USB to EIDE, SCSI-2, and Ethernet adapters are also made.

Q7: I'm planning to add USB to my system, buy a system with USB, or buy a USB device. What should I look for?
A7: The USB-IF does not endorse specific products, but you can search the USB-IF's product list to find out what's out there. As always, however, it's wise to look for products made by manufacturers who have invested the time and effort to required to build reliable products. In particular, many people have become quite frustrated with USB devices that are only supported under one class of host controller or with motherboards which, while having the chip needed for USB, didn't come with any USB ports to plug USB devices into. As a result, the USB-IF does suggest looking for USB devices that will work with any host controller (both UHCI and OHCI) and for motherboards which come with USB ports.

Q8: So how does USB compare to IEEE-1394)?
A8: While the two serial buses seem similar, they are intended to fulfill different bandwidth and cost needs. 1394 can move more data in a given amount of time, but is considerably more expensive than USB due to its more complex protocol and signaling rate. Applications that are best suited for 1394 are disk drives, high quality video streams and other high bandwidth applications; all higher end consumer devices. USB is appropriate for middle and low bandwidth applications such as audio, scanners, printers, keyboards, and mice.
USB and 1394 are complementary technologies. 1394 is for devices where high performance is a priority and price is not, while USB is for devices where price is a priority and high performance is not.

The above  information was found at

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