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USB Info: Frequently Asked Questions

So what does USB really do?

Q1: How fast is USB?
A1: High speed USB products have a design data rate of 480 Mb/s. Full speed USB devices signal at 12Mb/s, while 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-1: 5MBYTES/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: 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: Can I use game port, serial, parallel, or PS/2 devices on USB?
A3: 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.

Q4: Will using a USB to serial or parallel adapter free up interrupts on my system?
A4: Yes, but you'll probably have to manually disable the serial or parallel ports in your BIOS before something else can use the IRQ.

Q5: What about EIDE, SCSI and network adapters?
A5: USB to EIDE, SCSI-2, and Ethernet adapters are also made.

Q6: I'm planning to buy a system with USB, or buy a USB device. What should I look for?
A6: 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 required to build reliable products. In particular, USB-IF supplies a certified logo to and lists products in an integrators list that pass testing that demonstrates significant compliance to the USB spec. These products are good choices to minimize the risk of compatibility problems between products.

Q7: So how does USB compare to IEEE-1394?
A7: While the two serial buses seem similar, they are intended to fulfill different market and cost needs. 1394 has the potential to 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 high quality consumer or professional video streams and other high bandwidth entertainment applications; all higher end consumer devices. USB is appropriate for  high and low bandwidth computer peripherals such as mass storage,video, audio, scanners, printers, keyboards, and just about any peripheral.

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