Electronic Design

PC-Based Instruments: Not Just Hype

This article is written in response to the criticisms of PC-based instruments made in "PC-Based Instruments Meet Traditional Instruments", a POV column by Grant Drenkow (March 1, 2004, p. 16, ED Online 7371).

It is true that specialized measurements require traditional instruments. However, PC-based instrumentation systems-comprising a Pentium PC or controller, PCI or PXI bus, graphical software, and instrument modules-have far more capabilities than most racks of traditional instruments.

When asserting that "PC-based instruments account for less than 2% of the test and measurement market," the author does not provide an information source or indicate if he was referring to sales volume, number of products available, or unit volume. Approximately 1000 PXI products are available from 60 vendors, most of which can be located at www.pxisa.org. Presumably, even more plug-in data acquisition (DAQ) boards exist. If we assume 2% is a sales volume metric from a credible source, the assertion underscores the low cost of PC-based versus traditional instrumentation.

Faster Processor. It is misleading to say, "Faster processors improve analysis speed but provide no gains in the speed, accuracy, or resolution of the measurement hardware." Most PC-based instruments, optimized to take full advantage of the PC processor, perform faster digital filtering and signal processing that translates to faster overall measurement throughput. The fastest digital multimeter today, the National Instruments (NI) PXI-4070, makes 6.5-digit resolution ac and dc voltage and current measurements at 100 samples/s. Moreover, the more analysis your application requires, the more you will benefit from a PC's fast processor. This is a direct result of the author's statement.

GPIB Bottleneck. The more data that moves between instruments and host PC, the more beneficial PC-based instruments are in terms of bus throughput. GPIB (1.5 Mbytes/s) is the bottleneck in many automated test applications, forcing the adoption of VME/VXI (40 Mbytes/s) followed by PCI/PXI (up to 132 Mbytes/s). Today's PC buses are an order of magnitude faster than previous buses, which were adopted specifically to bypass the GPIB bottleneck. VXI and PXI also include a timing and synchronization bus, unavailable with GPIB, to synchronize multiple instruments from multiple vendors.

Higher Performance for Less Cost. What traditional instruments today compete with plug-in multifunction DAQ boards? A multifunction DAQ board can combine eight channels of 16-bit differential analog inputs, two channels of 16-bit analog outputs, 8 bits of TTL-level digital I/O, two channels of 24-bit, 20-MHz counter/timers, and a bus throughput up to 132 Mbytes/s-for less than $2000, PC included. If such an instrument exists, it most likely has a fraction of the capability and versatility of the PC and multifunction board combination for a much higher cost.

The author is astute in observing that a $500 plug-in board "doesn't contain the circuitry and triggering needed to make real-time measurements and complex data reduction and analysis." PC-based instrumentation uses PC software, not the instrument, to perform data reduction and analysis. If high bandwidth is required, consider a higher-performance PC-based digitizer, such as LeCroy's PXD-1020 series (1 GHz).

PC Software. The article echoes several ancient myths of traditional instrument users. "The PXI approach can't emulate a traditional instrument, such as a scope, where you can change range, adjust the cursor, zoom in on data, and make complex measurements with the turn of a knob." Most PC-based instrument manufacturers provide driver software that includes a "soft" front panel or examples that allow users to control the instrument immediately, with no programming required. Figure 1 and Figure 2 show two virtual oscilloscope examples.

Contrary to the author's opinion, it does not "take a team of experienced programmers and measurement experts to develop a complex program, send strings of commands to various PXI or PCI modules, bring back megabytes of data, run the data through measurement algorithms and decimation routines, filter the data, and store and display the results." Users develop software called "virtual instruments" using the high-level functions provided in the instrument drivers, not low-level strings of commands. The instrument driver functions return megabytes of data as scaled arrays. Measurement and analysis routines are included in graphical software development environments like NI LabVIEW, which contains many man-years of measurement expertise that enables users to customize PC-based instrumentation to their exact needs.

Indeed, the very small form factor of PC-based instruments may preclude some specialized instruments requiring high power or large components. However, most mainstream electronic test instruments, such as 6.5-digit digital multimeters, 1-GHz digitizers, 100-Msample/s signal generators, 100-MHz digital I/O, and a variety of switch modules are all available in a PXI form factor. Additionally, plenty of specialized instrumentation for high-frequency measurements, image acquisition, motion control, boundary scan, and bus interface testing, as well as low-cost multifunction data-acquisition modules, are all available in the PXI form-factor. Most modern automated test systems combine PC-based instruments for mainstream measurements, and traditional instruments for specialized requirements. Maximizing the PC-based instrumentation will optimize the speed, flexibility, integration, size, and overall performance of the measurement system.

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