The magazine of the Melbourne PC User Group The Care and Feeding of USB Paul Zucker © |
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The U in USB stands for Universal, and that's a clue to what it is (or should
be). It's a way to connect all sorts of things to your PC as Paul Zucker
explains. |
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Early PC's used a number of in and out ports to connect external devices. There
was the
printer port (also called the parallel
or Centronics port), the serial port (in
a couple of sizes) and the keyboard
port. Later came the PS/2 keyboard
and mouse ports. To a great degree
these have all been superseded
by the USB (Universal Serial Bus)
port, and most new computers
have at least four of these. A similar system is the FireWire port (also called 1E1394 and i-Link) and a partial alternative on Mac computers was the ADB (Apple Desktop Bus) for connecting devices such as keyboards. But once Apple started putting USB ports on all its PC's ADB was dropped. FireWire became more specialised and most PC users don't need it. Like the earlier parallel and serial ports USB is a bidirectional system. That is, it can carry data in both directions. It effectively combines both data and control interfaces in one simple connector. It also provides some electrical power, though this may be more of a nuisance as we'll see later. |
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USB capability has to be built into the operating system, but it's "plug and
play" which means that within reason you can connect and disconnect devices
without having to reboot the system. This is also called "hot-swapping". Many
USB devices are also able to operate without the need to install
the plugs.
The standard USB plug (and socket) is called the "A" plug. This is the one found
on the PC and hubs. It's designed to be easy to connect
specific device drivers, which is another advantage. Devices that use USB
connectivity include
printers, scanners, graphics tablets, cameras,
MP3 players, card readers, PVRs, other computers and so on. In fact you probably
have around
a dozen USB devices in your home today.
USB version 1 was introduced in 1995 but it wasn't until the late 1990's that we
started to see USB devices on the market. USB version 2 was introduced in 2001,
offering much greater data speed and it has backwards compatibility with USB v1
devices.
A USB port (called a controller in USB-speak) can feed into a hub which is in
effect a repeater and multiplier. Theoretically up to 127 devices (including
hubs) can be connected to a single port. There can
be up to five levels of hubs.
But note that USB is only rated to work up to a distance of five metres, so even
if you put a hub every five metres, you'll still only get a maximum of 25
metres,
so USB is not the way to go for connecting distant devices.
Why is the limit 5m? Because the signal timing won't allow any longer delay than
you get on a 5m cable.
The controller is also known as the "root hub". Each USB device can combine a
number of functions on that single connection, such as in a multifunction
printer/scanner/ fax. USB really is a clever system.
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These are two distinct USB plugs. The A plug on the left is found on the computer or hub. The B plug on the right is found on the connected equipment. The cable uses different plugs at each end because in general you should not connect two like devices together (eg two PCs) |
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The Plugs
The standard USB plug (and socket) is called the "A" plug. This is
the one found on the PC and hubs. It's designed to be easy to connect
and impossible to connect incorrectly, but most of
us old buggers still have to try both ways to get
it to go in. However,
once the connector
goes in you know it
by feel, so the design
isn't too bad. Another advantage is that there's
no clip or special action needed to unplug the lead.
You can use a USB extension cord (male and female ends)
but just remember the 5m
length limit. It's very hard to find
USB connectors from different
manufacturers which DON'T fit, which
is another advantage or the design.
I'm going to rant a bit now. Unless
a USB cable is very cheaply made,
there's no need to buy special USB cables. They're all electrically the same and
work the same. Buying a USB "Monster Cable" is sheer foolishness.
Even buying a "USB2" cable over a cable that's just labelled "USB" is useless,
Buy by price, not fancy packaging. Despite what some people will believe, you
can't get a better sound or picture
through a more expensive USB cable. Sorry.
There are special USB cables that will connect two computers or devices such as
PDA's
together. These are called USB On-The-Go.
Power
The USB standard says that a USB port should provide a power supply of 5V with a
current
draw up to 500mA. This power is available to
the connected device, such as a hard disk. But it isn't much power - just 2.5W
maximum.
And that power has to come from somewhere - just remember that next time you
plug in a USB-powered coffee warmer. Connecting those "cute" USB-powered
gadgets is especially silly on a notebook where power is limited already. The
other problem with power is that when you connect an unpowered hub to a USB
port, the
500mA gets shared by all
devices connected to the hub.
This is a common source of problems with PC users.
The solution is to use a
powered hub. That is, one that has a plug-pack connected to it. This will ensure
that each port on the hub gets the full 500mA.
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When you plug a single USB1 device into a USB2 hub, it forces the hub to run all
connected devices at the much slower USB1 speed. Ahah! Is that what's been causing that problem! |
As you may have seen, some devices use a double-ended USB cable (two plugs on
the PC end) to double the available power to the device.
There are some exceptions to the power
rule and this will become more common on
newer motherboards. For instance, some
devices may be able to draw up to 1.5A. If you're interested, check the USB specification on-line as this is quite an intriguing subject (to some people). While you're on-line, it's also fascinating to read about the related technologies such as PictBridge and Wireless USB and other new ideas. Check www.usb.org and the Wikipedia entry on USB. |
Note: Paul Zucker© Permission to copy or
quote extracts from this article may only be done with the written permission of
the author.
Reprinted from the September 2008 issue of PC Update, the magazine of Melbourne PC User Group, Australia
