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USB Uncovered

USB (Universal Serial Bus) is a system for connecting devices to personal computers. USB became the standard for consumer devices nearly overnight. Apple adopted USB as the only peripheral connection to their iMac line of computers, and USB connections were built into nearly every PC on the market. Consumer device manufacturers realized that they no longer had to sell devices with separate Mac and PC versions. USB is easy to use, fast, and cheap to manufacture: a gem of simplicity in an ever growing mountain of computer complexity.


USB Types


There are two types of USB signals: version 1.1 and version 2.0. While you must use cables that are compatible with your signal type, USB 1.1 is compatible with 2.0 cables. Moreover, most cable manufacturers have stopped making 1.1 style cables, so nearly all cables sold today are already 2.0.


The problems come up when trying to match connector types. Here is what they look like:



USB Type-A Male
USB Type-A Male
USB Type-A Female
USB Type-A Female
This is the most common USB connector found on the backs of computers, mice, keyboards, mp3 players, and some hubs.

USB Type-B
USB Type-B
This type of connector is typically found on the back of printers and upstream ports of some USB hubs.

USB Type Mini-B 4-pin
USB Type Mini-B 4-pin
This is commonly used with consumer electronic devices such as cameras, mp3 players, and PDAs. This version is missing a particular pin for power. Some devices require power from the USB connector, and will not function properly without that power. Please consult the documentation to see which you need.

USB Type Mini-B 5-pin
USB Type Mini-B 5-pin
Much like its 4-pin cousin, this cable is used for devices such as cameras, mp3 players, and PDAs. This version has the pin for power connected. Some devices may be damaged if they are not expecting this pin to be connected, so please consult your products manual to determine which you may need.


Going for Speed


USB version 1.1 (designated "Full Speed USB" by the USB consortium) is by far the most common for today's computers and computer components. USB 1.1 can run a single channel at 12 Mb/s (1.5 MB/s), or it can be broken into 8 subchannels at 1.5 Mb/s (187.5 KB/s) each. USB version 2.0 (designated "High-Speed" by the USB consortium) has a maximum bandwidth of 480 Mb/s (60 MB/s). USB 2.0 is one of the fastest consumer data transfer systems on the market. To give you an idea how fast it is, here is a list of data rates for different types of connections:


TypeMillion bits/secMillion Bytes/sec
Serial Port:0.115Mb/s0.0143MB/s
Parallel Port:0.920Mb/s0.115MB/s
Broadband Internet:1.5Mb/s0.187MB/s
10Base-T Network:10Mb/s1.25MB/s
USB 1.1:12Mb/s1.5MB/s
PCP/EPP Parallel:24Mb/s3MB/s
100Base-T Network:100Mb/s12.5MB/s
IDE ATA 33:264Mb/s33MB/s
IEEE-1394 (aka. FireWire, iLink):400Mb/s50MB/s
USB 2.0:480Mb/s60MB/s
SCSI Ultra-Wide2:640Mb/s80MB/s
IEEE-1394 800 (aka. FireWire 2.0):800Mb/s100MB/s
1000Base-T Gig. E.:1000Mb/s125MB/s
IDE ATA 166:1328Mb/s133MB/S
SCSI LVD Ultra-Wide 320:2560Mb/s320MB/s

  • Units:
  • bit = 1 or 0
  • Byte = 8 bits
  • Kb = 1,000 bits
  • KB = 1,000 Bytes
  • Mb = 1,000,000 bits
  • MB = 1,000,000 Bytes

A USB Bridge Too Far


The major complaint about USB is the length limitation of 5 Meter (16.4 ft). There are several solutions to this problem, and most of them are device specific. There are some general solutions, but they tend to be cost prohibitive. Here's a few options along with their benefits and drawbacks.


Optically Isolated cables


A USB signal can be converted into an optical signal, run along a fiber optic, and converted back on the other end. With the proper fiber, you could conceivably run this up to a kilometer (or longer if the optical switching system is faster than the USB reflection). However, this will not work for bus powered peripherals (devices that are powered by the USB cable), and the cables are extremely expensive. A 30 Meter (~100 ft) cable will commonly sell for $200; which is often more than the device that you're trying to control.


USB Network Print Servers


The most common device to want to have far away from a computer is a printer. A USB print server is a small network device about half the size of your mouse that allows you to hook your USB printer up to your home or office local area network. The server is configured once with a PC, and then any computer on the network can print to that printer. Look at USB 2.0 1-Port Print Server 10/100Mbps Fast Ethernet for more information.


USB Repeater Cables


Repeater cables use the same chips as USB hubs, but they have a single input and a single output. A single repeater cable will allow you to extend your device up to 10 Meters (32.8 ft), and you can chain up to five repeater cables together. However, running five of them together won't work for bus powered devices, is incompatible with some devices, and often only works at a reduced speed with intermittent operation. While some have been successful with five, you probably shouldn't use more than two repeaters for one device.


Simple Extension Cables


The best solution can also be the most simple. If you are trying to extend a simple device like a mouse or keyboard, you can often just use a simple USB extension cable (male on one side, and female on the other). While this will not always work, USB cables are very inexpensive it's sometimes worth just trying.


Other Common USB Problems and Their Solutions


Old Computer, New Trick


If you have an older computer that doesn't have USB ports, then you may still have the option of installing a USB controller card. They are simple to install, reasonably inexpensive, and make for an easy way to update your older computer.


Old Trick, New Computer


If your computer is so new that it does not have a serial port on it and you have an older serial device (GPS unit, thermal printer, serial device controller, etc) that you wish to use, then you can use a USB to DB9 converter. You can set these devices to simulate any COM port on your computer to accommodate existing software, and you're up and running in minutes.


Two Computers, One USB Printer


While this problem can be solved with a USB Network Print Server as described above, not everyone has a network in their home. An Manual Sharing Switch will allow either computer to print to the printer and simply switch between them. However, if both printers try to print at the same time, there maybe problems with the print job, so the network solution may be preferable in some cases.


PS/2 (MiniDin6) Woes


Much like the serial example above, if you have a PS/2 mouse and keyboard and would like to use them on your USB computer, then you can use a PS/2 Active Adaptor. This acts as a small PS/2 controller and will work with any brand of any keyboard and mouse.







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Last Updated: July 25, 2014