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Alan
Lowne, Saelig Co. Inc.
USB Basics
The Universal Serial Bus (USB) was initially created to connect peripheral devices, such as keyboards and mice, to PCs. However, USB has proven valuable for many other tasks, including measurement and automation. USB 2.0 has also increased data rate from USB1.1’s maximum 12 Mbps to 480 Mbps. Additionally, the USB host automatically detects when a new device is added, finds the device identification, and configures the appropriate drivers. USB supports Plug and Play so that the instrument is immediately recognized and configured on plugging in. Up to 127 USB devices can run simultaneously on one USB port. By adding low-cost powered hubs, even more ports can be added to a USB host, creating connections for more peripheral instruments. USB provides up to 500mA of current to the peripheral which can often be used to power the instrument, eliminating the need for and external power supply. The maximum cable length allowed is about 15 feet for USB devices, but this can be increased up to 150 feet with USB extenders.
USB modules offer good noise immunity, with performance benefits for noise-sensitive measurements. USB sends data through either isochronous or asynchronous data transfers. With isochronous transfers, USB protocol guarantees bandwidth with time-based delivery of data packets. Isochronous transport makes sure that a transmission is completed within a certain time, but it does not guarantee that the transmission is received error-free. The USB protocol ensures that devices that have requested isochronous bandwidth are given a predetermined number of data packets in each frame. Data-intensive applications often demand isochronous bandwidth.
USB-based instruments have many advantages, since they use your PC’s resources for GUI, control, display, print and storage (save the expense buying those capabilities again!) They use external components to do the sampling and other I/O. They make ideal lightweight take-along devices for putting in your laptop case for field trips or show-and-tell at customer sites. They are easy to update too with new software features as they become available, which are often downloadable at no charge from the manufacturer. It is easy to expand the number of USB ports available by adding USB hubs which are readily available at low cost. USB is fast and allows for a cable length of up to 5m between the instrument and the PC.
What’s out there for Test and Measurement?
You can buy USB-based test-gear for almost any task: audio test, calibrators,
dataloggers, function and signal generators, meters, logic analyzers, video microscopes, oscilloscopes, temperature/humidity meters, test jigs, etc. USB-connected test-gear often draws its power from the USB connection itself, but be careful about grounding and isolation: if grounds are common then not only could your laptop fry when your test-gear is overloaded, but low-level measurements may have a ground offset voltage. Always ask if a supplier will let you try the equipment on a sale-or-return basis for your application on your bench.
Selecting the right USB instrument for your needs depends on your budget and requirements. For scope adapters, the number of channels and the sampling rate may be a selection factor. Good waveform reproduction is only achievable at the sampling rate divided by at least 4, and more likely by 10. So a scope adapter sampling at 100 MS/s will probably have enough samples to show an adequate reproduction of a 10 MHz signal. An exception is for repetitive signals, which by multiple sampling allows you too see much higher frequencies if the input amplifier will pass that bandwidth. For instance, Pico Technologies’
PicoScope
3204 is a dual-channel scope adapter sampling at 50 MS/s but states it can simulate sampling rate of 2.5 GS/s for repetitive signals.
National Instruments, for instance, offers a range of USB-based devices; including a 15 MHz digital oscilloscope (NI 5102 for USB), data loggers (NI 4350 for USB), and digital I/O devices (DAQPad-6507/8). For USB temperature sensors, ask yourself if the device has to be connected to a PC or can it run standalone. What accuracy does it need?
Before choosing a USB data acquisition module, consider the application and the environment. It’s also wise to ask yourself if the data acquisition module might be susceptible to electrostatic discharge (ESD), lightning, or power surges from motors, switching devices, or other equipment? Are there voltages that have different ground potentials? Isolation may also be required.
Websearch
Check out www.besttest.com and other
engineering sites for a multitude of USB products such as:
audio test/noise and vibration: http://www.mpihome.com/
dataloggers: www.picotech.com www.geminidataloggers.com
function and signal generators: www.keithley.com
meters: http://www.gamma-sci.com/
logic analyzers: www.cleverscope.com
video microscopes: www.elmousa.com
oscilloscopes: www.cleverscope.com www.tiepie.com www.picotech.com
temperature/humidity meters: www.picotech.com
USB Isolators: www.amplicon.co.uk
USB protocol analyzer: http://www.saelig.com/pr/ex200provisualusb.htm
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