Which is the better vehicle for driving—a sports car or a mini-van? That all depends. A sports car gets you to the restaurant very quickly but probably won’t hold the five friends you invited to ride with you.
The same is true with data acquisition systems. The best system is determined by its intended application. And just as you would check out the market before deciding whether to buy a sports car or mini-van, you have three major choices—GPIB instruments, PC plug-in cards, and VXI systems—to look at before you select a data acquisition system.
GPIB Instruments
The instrument bus (GPIB) was designed in the 1970s as a standard communications protocol between instruments and computers. Today, it still is the most popular choice when connecting computers to instruments, even though the typical transfer rates are below 500 kB/s.
GPIB-based data acquisition systems have flexibility designed into the instrument. Often, this takes the form of proprietary plug-in modules in the back of the instrument. Communications with these plug-in modules are made through ASCII commands and GPIB subaddresses.
GPIB instruments range from relatively inexpensive to very expensive, depending on the sophistication of the measurements and the number of options selected. They are best used in applications where accuracy is important but high-speed data transfer to the computer is not.
PC-Based Instruments
PC-based instruments became popular in the late 1980s when hundreds of companies began producing various types of data acquisition plug-in cards for the IBM PC, first for the ISA bus and later for EISA. The market for these cards grew as users found them convenient for adding a few measurements to their computers. As computers increased in speed, the backplane was redesigned to accommodate the Pentium processor.
The Peripheral Component Interconnect (PCI) bus was created in 1992 with a speed of 132 Mb/s. As a result, many PC-based instruments now are designed to fit this backplane.
PC-based instruments have an advantage in cost over other architectures. They use the existing computer backplane for power and data transfers. High-speed plug-in card-to-computer-memory data transfers are quite natural.
In some cases, PC plug-ins can make these high-speed transfers directly to memory without CPU intervention. For a few measurement channels, these cards offer the lowest cost alternative.
The disadvantage of a PC plug-in card is the computer environment that surrounds it. This environment never was designed for sophisticated and accurate instruments that need lots of power, high-performance cooling, and carefully designed EMI/RFI shielding.
Wiring to the plug-in cards can be difficult, depending on the type of computer used. Most commercial computers have a limited number of slots, making it difficult to accommodate very many channels. Industrial PCs have more slots, but you lose the price advantage. In most cases, PC plug-in cards are used when high-speed, low-resolution measurements are needed.
VXIbus Instruments
In 1987, VXI was created as a standard by the major instrument manufacturers. It features regulated power supplies, powerful cooling, and tightly controlled EMI/RFI shielding.
A self-test ensures each module is checked out before operation. Well-designed software protocols guarantee proper bus arbitration.
Major data acquisition companies use VXI for their high-performance systems. Based on the popular European computer bus (VME), the VXI backplane is designed for 40-MB/s data transfers.
The VXI Consortium extended the specification to include high-speed card-to-card triggering and data transfers of up to 100 MB/s. It is not uncommon for sophisticated UNIX or Pentium computers to coexist along with high-speed, highly accurate instruments and switches.
Up to 13 cards can be plugged into a single VXI cardcage. VXI cardcages can be linked together to form data acquisition systems with thousands of channels. The design of VXI cards with wiring connections easily accessible is a popular choice for applications where the measurements change quite often. In time-critical applications, a VXI-configured system makes it easy to disconnect a terminal block with hundreds of points, change a card, and reconnect without time-consuming rewiring.
C-size VXI cards are roughly twice as large as PC plug-in cards and offer more signal conditioning. Data conversion is performed right on the VXI card. Using plug-on signal-conditioning daughter boards, multiple types of measurements can be performed on a single VXI card.
Using the 100-MHz VXI local bus, data can be transferred among plug-in cards for additional processing or to high-speed storage on another VXI card. The combination of high-speed buses, trigger lines, and intelligent VXI cards makes VXI the choice for real-time applications and those requiring more than a few channels.
Selection Process
Channel count probably is the first selection criteria for a data acquisition system. If the application only has a few channels, PC plug-in cards are the most convenient.
As the channel count rises, GPIB instruments may be more cost-effective since adding another PC, or one with more slots, can be quite expensive. As the application becomes larger, VXI and its fixturing solutions make it the obvious choice.
Accuracy also is a consideration. Both GPIB instruments and VXI are designed for accuracy. The deciding factor may come down to the number of channels or the types of measurements to be made.
Selection of a system based on speed can be a little tricky. An application that requires a high-speed burst of measurement data may find a good match with a PC plug-in card sitting directly on the backplane.
As the number of channels increases, a high-speed scanning A/D system, like those found in VXI, will be required to maintain the measurement speed per channel. A single channel of high-speed data collection and fast signal processing may work well with a PC plug-in. But as the number of channels and the sophistication of processing grow, a VXI card with multiple DSP processors may be the most cost-effective answer.
An application where real-time monitoring is important is possible with a PC plug-in card for one or two channels. However, an application that requires data collection from multiple channels at varying rates and then shifts to high-speed data collection based on an external event is possible only with intelligent VXI cards.
Conclusion
As you can see, GPIB instruments, PC plug-in cards, and VXI systems are viable choices for a data acquisition solution. Which one is best? That all depends on the speed, accuracy, and sophistication of your application.
About the Author
Grant Drenkow is the marketing programs manager for the Measurement Systems Division at Hewlett-Packard. He served six years in the U.S. Air Force as an electronic warfare officer. In 1983, Mr. Drenkow joined Hewlett-Packard where he has worked as a product support engineer, an application engineer, a regional sales engineer, and a product marketing engineer. He has a B.S.E.E. degree from the University of Nebraska and an M.B.A. degree from the University of South Dakota. Hewlett-Packard, Measurement Systems Division, 815 14th St. S.W., MS CU326, Loveland, CO 80537, (970) 679-3128.
Copyright 1997 Nelson Publishing Inc.
November 1997
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