Universal Serial Bus-equipped PCs, peripherals, and data acquisition systems finally are available. The upcoming 1.1 specification will refine the Universal Serial Bus (USB), and expanding software driver support will bring it to the mainstream. So what does this mean to you? Just plug in and find out.
There has been a lot of talk about the new USB—and for many good reasons. Launched at Comdex/Fall in 1995, USB pledged to make life easier for everyone using a PC.
About half of the PCs sold in 1997 were USB equipped, and nearly every computer now on the market includes a USB port. With Windows® 98, which features USB software drivers for a broad spectrum of peripherals, the new bus will become standard for average computer users and test-and-measurement engineers.
USB was designed to replace the confusing proliferation of ports on the back of our PCs with a bus that could serve just about every low- and medium-bandwidth peripheral imaginable. This would include digital cameras, video phones, scanners, monitors, keyboards, printers, joysticks, telephony and broadband adapters, and data acquisition systems. Developed by a consortium consisting of computer-industry heavy weights (Compaq, Digital Equipment, IBM, Intel, Microsoft, NEC, and Northern Telecom), the USB dream now is a reality.
USB offers unprecedented plug-and-play performance and eliminates the installation hassles often associated with many peripherals. With USB, the days of removing the cover of the PC to install new devices are over. There are no costly add-in cards or complex configurations, such as adjusting DIP switches, I/O addresses, DMA channels, and IRQ settings.
Soon, common USB devices—keyboards, mice, and monitors—will be configured automatically by the operating system of the PC when first connected. In some respects, installing a new USB-based peripheral will be similar in simplicity to adding a component, such as a VCR, camcorder, or a video game, to your television. With these components, you just connect the standard jacks, and the installation is complete. You don’t need to use different connector types or configure your television for each new component.
The same basic concept is true for USB-based peripherals. When a USB-based device is first installed, it looks for the appropriate software on the hard drive of the host PC. In most cases, USB peripherals will be recognized.
If the drivers are not found, a message requesting you to install the driver will appear on your monitor. In this scenario, completing the installation is only a matter of loading the software driver via CD-ROM, floppy disk, or on-line download. Once installed, peripherals can be hot swapped, meaning they can be added and removed without turning off or rebooting the host PC.
Through branching, a single USB controller can serve up to 127 peripherals simultaneously. This is accomplished with hubs that connect an upstream port to numerous downstream ports, creating a tiered-star multidrop network.
Any device can be plugged directly into the port of the PC or into a hub that can accept up to seven other peripherals or hubs. Up to 5 meters long, the USB standard cable features different upstream and downstream connectors to prevent miswiring. In addition to signal connections, the cable supplies power to peripherals, enabling keyboards, mice, and other low-power devices to operate without a separate power supply.
Performance
USB provides highly structured communications for maximum efficiency and reliability. The main mode of USB is 12 Mb/s; however, typical peripheral throughput is less than 1 MB/s. Devices such as mice that do not need the high speed of the USB can use its alternate 1.5-Mb/s mode to avoid the extra cost of high-speed components and shielded cables.
USB also defines four data transfer types:
Control transfers are primarily responsible for device configuration/command/status- type communications. When a device is plugged in, control transfers get things up and running.
Isochronous transfers commonly are used with devices that require specific bandwidths, such as audio devices. They reserve guaranteed bandwidths to keep peripherals with specific requirements operating.
Interrupt transfers communicate low-frequency data. They are designed for low-bandwidth devices, such as mice and joysticks, and enable the host PC to poll these devices for service information.
Bulk transfersare appropriate for large data storage and scanner devices. They send nonperiodic data in packets compatible with whatever bandwidth is available.
In addition to improved throughput and highly structured transfer types, USB provides critical operating information. For instance, you will receive a message suggesting another peripheral be shut down if the requested bandwidth is not available for the current application.
Emerging USB Products and Driver Support
Many manufacturers are announcing USB-based peripherals. Despite widespread USB hardware availability, limited software driver support for general peripherals has been shipping with the major PC operating systems.
Microsoft recently started supplying software driver support with its OEM Service Release Version 2.1 of Windows 95. Windows 98 includes a wide range of USB software driver support for mice, keyboards, monitors, and other commonly used peripherals.
Mainstream operating systems do not provide software driver support for data acquisition units. However, data acquisition system manufacturers have been and will continue to supply product-specific drivers for their emerging USB-equipped data acquisition test-and-measurement products (see sidebar).
Data acquisition system manufacturers have been quick to jump on the USB bandwagon because it is ideal for portable PC-based data acquisition. In particular, USB is well suited for ad hoc applications requiring quick setup and teardown, such as troubleshooting and field service testing. Examples include voltage and temperature measurements, where simple plug-and-play devices leveraging power from USB-equipped PCs allow quick installation and high-quality data acquisition.
USB Specification 1.1
USB’s 1.0 specification has been unchanged since January 1996, a sign of stability, but a 1.1 specification is in the works. According to the USB Implementers Forum, the new specification will conservatively refine this 12-Mb/s connection and eliminate rare error conditions or multiple error conditions that might occur concurrently. Supposedly, the main rewrites involve accelerating the process of enumerating devices on the bus and defining more boundary conditions.
Also expected from the new specification is an interrupt-out feature that will enable a host controller to periodically send data to a USB peripheral delivered in a certain amount of time. This will serve devices, such as joysticks with force feedback, that have some priority demands but do not require multimedia streaming.
The new specification is expected to be released soon. Manufacturers will have 12 months after that to comply with any revisions. Current USB devices will be compatible with the new 1.1 specification, but host computers will require new software drivers, and USB hubs will need to be upgraded.
USB and FireWire: Friends or Foes
Like USB, there has been a lot of talk about FireWire (IEEE 1394), a new bus with speeds of up to 3.2 Gb/s. People unfamiliar with bus issues might ask if the FireWire eventually will replace USB. Not likely. FireWire’s performance is an overkill for the low- and medium-speed peripherals that USB serves.
Instead of competing against each other, USB and FireWire will work together to meet the PC industry’s growing list of peripheral interconnectivity needs. USB serves common peripherals. FireWire, roughly 15 times more expensive for OEMs to implement than USB, supports less common, high-dollar peripherals requiring high bandwidth, such as digital video cameras, DVD players, and large-capacity storage devices.
FireWire features a tree topology that allows devices to connect to each other as long as there are no loops. A FireWire system supports up to 63 peripherals, and it features peer-to-peer connectivity to allow peripherals to talk to each other without intervention from the host PC.
Like USB, FireWire enables peripherals to be hot swapped. The bus will reset, reconfigure, and continue operating when a device is added or removed while the system is under power.
About the Author
Allen Tracht is the principal engineer at IOtech. Mr. Tracht holds electrical engineering and physics degrees from MIT and a master’s degree in electrical engineering from the California Institute of Technology. IOtech, 25971 Cannon Rd., Cleveland, OH 44146, (440) 439-4091, www.iotech.com.
Sidebar
Data Acquisition Finds a Front Seat on the New Bus
As the USB begins to roll, test-and-measurement product manufacturers are introducing a breed of USB-based devices that benefits from plug-and-play installation and low- to medium-throughput. New products include USB-based A/D modules, data acquisition and digital I/O systems, and voltage-and-temperature measurement instruments.
These USB-based products are cost-effective alternatives to data acquisition PC Cards and plug-in boards. Ideal for low-level signal measurements, USB-based instruments reside outside the noisy PC environment. They can be placed near the signal source up to 5 meters from the PC with a single cable or up to 30 meters using USB hubs as repeaters.
USB also provides both communications and power to the peripheral. For example, IOtech’s Personal Daq/55 Data Acquisition System permits single-cable connection and eliminates the need for an external power source. The 22-bit system measures voltage, thermocouples, pulse, frequency, and digital I/O. It features 10 single-ended or five differential analog or thermocouple input channels and 20 digital I/O lines and is expandable to 60 analog input channels.
Copyright 1998 Nelson Publishing Inc.
July 1998
