Imagine adding data acquisition capabilities to your PC as easily as plugging in a mouse or a
keyboard. Just connect a compact data acquisition module to your computer (you don't even
have to power-down), install the supplied software, and connect your sensor directly to the
module. In minutes, and with state-of-the-art accuracy, you're capturing data: temperature,
pressure, sound level, whatever you need. And you have done this without programming, without
opening up your PC, without even thinking of IRQs, board configuration, power requirements, or
A pipedream? Not any more. This convenience and power are available today, thanks to the
USB, a serial expansion bus, and to a growing range of USB-compatible data acquisition function
modules. Together, USB bus and modules greatly simplify the way people perform data
acquisition on PCs.
USB: A Truly Universal Bus for Computer Peripherals
Today, virtually all desktop and laptop PCs on the market provide USB ports. USB ports for
existing PCI Bus machines can easily be added. Equally important, these USB ports have full
software support under the most popular personal computer operating systems, including
Microsoft® Windows® 2000 and XP.
The USB was created by a group of manufacturers (including Intel , Microsoft, and Compaq) to
simplify the connection of external peripherals to personal computers. The USBs Web site
(http://www.usb.org) has detailed information on the features of USB.
Are one or two USB ports enough? Expandability is another USB strong point. Up to 127 devices
can connect to a single port simultaneously, using expansion hubs and cable assemblies that are
widely available at low cost.
Fig. 1: USB Bus Configuration Overview
USB 2.0 Improvements
The first specification for the USB, version 1.1, featured 12 Mb/s (megabits per second) data
throughput, which is fine for moderate-speed applications, including most data acquisition uses,
but not fast enough to support video, imaging, and highest-speed data acquisition applications.
This technical limitation was addressed with a new version of the USB specification. In April
2000, the USB 2.0 Promoter Group released the final USB 2.0 specification. USB 2.0 exceeds
the previous versions speed barrier with 40 times the bandwidth: 480 Mb/s. Advances such as
larger packet size and new transfer modes make high-speed transfers more efficient. As higherperformance
USB peripherals become more widespread, 2.0s additional bandwidth will make it
feasible to connect more USB peripherals, supporting multi-tasking software operations without
slow-downs. USB 2.0 (Hi-Speed USB) is fully compatible with USB 1.1 (Original USB), and uses
the same cables and connectors.
Since ease of use was primary in the creation of the USB, many features have been designed in
to make USB devices among the easiest to install. To begin, USB devices are true plug-and-play.
There is no setting of address lines, interrupts, or configuration jumpers. The host PC
automatically identifies a peripheral when it is plugged in, and searches for the software
necessary to operate it. The first time a device is used, the user is prompted to install the drivers
and application, a process that takes just a few minutes. Ease of installation means USB
peripherals can be shared between computers, even between laptop and desktop models.
USB devices are also external to the computer - there is no need to open up the computer to
install them. This external location also provides performance benefits for noise-sensitive devices
like data acquisition peripherals: They are away from the computers noisy motherboard and
power supplies, and closer to the sensors they will be measuring.
Hot-swappable USB devices are designed to be installed or removed while the computer is
running. Peripherals no longer need to clutter the workspace when they arent needed. Just plug
the device in, use it, then remove it when done.
Peripherals can either be powered by the USB or from an external power source. Being powered
from the USB is most convenient, but, since only limited power is available, a device must
typically be designed from the ground up for USB operation.
USB devices can plug directly into a USB port, located on the PC, in powered hubs that can be
standalone, or built into monitors and computer peripherals.
Fig 2: USB Connections
Connection to the PC is easy, too. A single, low-cost cable (from 1 to 5 meters long, and often
supplied with the peripheral) carries power and signal connections to the USB device. Since all
devices use the standard cables, USB users dont have to hunt down expensive, hard-to-find
adapters to change the pin configuration or gender of connectors before they can use their
peripheral. USB cables are available off-the-shelf at computer and office-supply stores.
USB devices for data acquisition may also include removable terminal blocks that conveniently
handle all user I/O connections. This saves time and money: No external screw terminal panel is
Perhaps the most profound benefit to the USB is the confidence it inspires in PC users of all
levels of experience. Anybody who can operate a PC can install a USB peripheral and have it
running in minutes.
For data acquisition users, this benefit can be enormous. No longer does the peripheral gather
dust until a computer expert can install it, or a busy scientist can take hours or days to figure out
how to configure and program it for the task at hand. Because USB devices work directly with
most laptops, even the most sophisticated data acquisition applications come out of the lab and
into the field.