When evaluating a PC-based data acquisition (DAQ) system, the current state of technology leaves us facing a choice between the industry standard architecture (ISA) bus or the newer peripheral-component interconnect (PCI) bus. Each technology has some compelling arguments in its favor; but for data acquisition, a good case is stacking up for PCI.
ISA Explained
Virtually all IBM-compatible PCs made before the Pentium were based on the ISA (IBM’s PC AT) bus. This asynchronous bus architecture uses 16-bit addresses and an 8-MHz clock and handles a maximum data throughput of 2 MB/s to 3 MB/s (Figure 1).
ISA bus peripherals are excellent for many applications and have served us well through the recent stages of PC evolution. But as PC technology continues to advance, a growing number of devices and applications push data transfer requirements beyond what ISA is designed to handle.
Across the ISA bus, applications can move a maximum of 400 kS/s. That is, no more than 400,000 data samples can be transferred across the bus, either to or from memory, each second.
When ISA peripherals begin to push the bandwidth limits of the ISA bus, you begin to pay either for memory or for time such as system delays. Data that cannot be sent immediately across the bus as soon as it comes in must be stored locally or stalled—or it is simply lost.
Enter PCI
The PCI mezzanine bus architecture, now standard equipment on all Pentium PCs, is a processor-independent bus specification that allows peripherals to access system memory directly without using the CPU. Not only does this free up the CPU to service other application calls, but PCI users also can simultaneously acquire data to memory and analyze existing data in real time, all while communicating with other functions on the network.
More importantly, PCI peripherals running asynchronously can send data along the 32-bit bus at a rate of up to 132 MB/s or 66 MS/s (Figure 2). This enables the PCI bus to support even the latest high-bandwidth data-intensive I/O applications, such as 3D, HDTV and enhanced video.
Using PCI’s bridging capabilities, multiple PCI buses can be connected with off-the-shelf PCI expansion hardware. This is done via a PCI-to-PCI bridge chip, which overcomes capacitive load limitations and expands the number of plug-in slots. To achieve this kind of expansion with ISA, you would have to add more PCs to your production setup.
A board plugged into a PCI slot carries its own configuration information in software. You do not need to set any jumpers or identify any base addresses—a common headache with ISA. Not only does this ease installation and use, but because all hardware settings can be controlled in software, you also can easily customize the configuration of your system at any stage of the operation. Table 1 lists the features and performance capabilities of the ISA and PCI bus architectures.
ISA in DAQ
In the face of the many advantages of the PCI architecture, why would anyone choose to go with an ISA-based DAQ system? First, many users implementing DAQ applications today already have ISA-based PCs in-house and must weigh the benefits of PCI against the added cost of purchasing a Pentium system.
Secondly, certain low-end, single-function DAQ applications will not necessarily benefit from the extra bandwidth performance offered by the PCI bus. As long as the PC runs as a “dedicated system” when performing DAQ, some applications will never push the bandwidth limitation of the ISA bus. Such dedicated systems often serve as single-function systems, performing only data in, without DAC output or control functionality.
For straight acquisition, ISA’s 400-kS/s bandwidth clearance can easily be enough. But since DAQ applications often require bi-directional data flow, that bandwidth is quickly consumed and application performance suffers.
For example, an application acquiring data from a laser micrometer at 300 kS/s, and then controlling a table that moves the item beneath the micrometer at 200 kS/s, will quickly consume all available bandwidth and generate an unstable control loop. Dropping data bits forces the application to sample data at a lower rate than the data is coming in, resulting in unstable or inaccurate readings.
ISA Is Secondary Bus
Certain users, although upgrading their systems to Pentium and PCI, will consider plugging the ISA-based DAQ board into the new systems. Although most PCI-based systems have been developed with a PCI-to-ISA bridge for this purpose, the performance of the ISA board may be compromised going into a PCI system.
On PCI-based systems with ISA slots, the ISA is a secondary bus, receiving secondary treatment when other peripherals are polling the bus controller. For example, during a hard-drive or video-card update, the PCI-to-ISA bridge becomes inactive for an indeterminate amount of time, cutting off the data flow to and from the ISA peripheral. Without expensive on-board memory buffers to warehouse data, high-speed memory transactions will inevitably result in data loss.
PCI: Ideal for DAQ
PCI’s many performance and functional advantages are critical to DAQ. As DAQ has traditionally pushed the limits of system performance, dedicated systems for DAQ have become a pervasive mindset in the industry.
The emergence of PCI-based systems is about to change that, promising to open up the number of PC-based DAQ applications. Inherent design features of the PCI bus that boost performance and productivity in DAQ include much faster bus speed, ease of installation and configuration, greater expandability and guarantee of future support.
Many users are moving DAQ applications to the PCI bus, primarily for higher throughput. Since performance in many DAQ applications is directly dependent on throughput, the bandwidth capabilities of the bus correlate directly to testing accuracy.
TCC Industries, a manufacturer of cellular phone accessories, recently migrated all of its testing PCs to PCI systems to achieve a higher degree of accuracy. The company must test and grade its high-sensitivity microphones using rms analysis of voltage output, where extremely high-speed acquisition is critical.
Initially, TCC Industries ran two test systems and found that its PCI system was registering more accurate results. The non-PCI system was dropping data points because of bus bandwidth limitations—a single millisecond separation in signal pick-up produced an inaccurate reading. With the PCI setup, testing time was reduced to 5 s per unit for a complete test, compared with 12 s to 16 s each with the non-PCI setup.
Processor independence of the PCI bus also enables DAQ applications to simultaneously gather and analyze data as well as send control output to an external device or application. With the multitasking capabilities of the PCI, you can acquire data into file A and run statistical process control on file B at the same time you are downloading that data through the local area network.
Omnirel, a manufacturer of power modules for military and industrial products, uses PCI-based DAQ systems for its throughput and multitasking capabilities. The company puts up to 120 power modules on line at any one time to perform rigorous electronic stress testing using PCI-based DAQ boards from Data Translation.
Some Omnirel modules control 2,000 V/1,400 A devices, making precision and repeatability critical. If the waveshapes are not uniform or if any part of the waveform is missing, it can destroy the device. Omnirel runs tests firing all the units at 10 Hz over a 24-h period, taking up to 1,000 samples of each waveform.
Conclusion
PCI has gathered significant momentum in recent years and shows no signs of letting up. On the other hand, the era of ISA is quickly coming to a close. Most new PCI systems are still manufactured with ISA add-in slots, but future systems will have fewer and fewer of these slots, until they ultimately cease to exist.
The PCI bus offers many performance enhancements that make it ideal for high-bandwidth applications and is sure to be a significant step forward in PC-based DAQ. While you will pay more for PCI-based PCs, the performance benefits realized in DAQ applications are worth the investment. With plenty of room for growth of the PC, the PCI bus gives you the safest and most robust platform on which to build DAQ applications.
About the Author
J. Chad Stalker III, who joined Data Translation in early 1995, is the Product Marketing Manager for the company’s Data Acquisition Product Line. Previously, he was a Manufacturing Engineer at General Scanning and a Manufacturing Process Engineer at Raytheon. Mr. Stalker graduated with a B.S. degree in mechanical engineering from the University of Massachusetts. Data Translation, 100 Locke Dr., Marlboro, MA 01752-1192, (508) 481-3700.
Table 1
Bus
Data to Memory
Data to Display
Bus Priority
Plug ’N Play
Data Size
PCI
80 MB/s
80 MB/s
80 MB/s
#1
Yes
64 bits maximum
ISA
2 to 3 MB/s
2 to 3 MB/s
2 to 3 MB/s
#2
No
16 bits maximum
Copyright 1996 Nelson Publishing Inc.
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