Putting Your Data on the Right Bus

Lately I have been building, debugging, and using different data acquisition (DAQ) systems for various purposes. In the course of this work (especially the debugging!) I started wondering about some of the "available options"—a code phrase for "How would I have done this if I had known what kinds of problems I would be in for with my present design?"

In the bad old days, there weren't many options in PC DAQ. You sent large sums of money to the vendors and they sent you ISA-format (or EISA if you were leading edge) cards that you put into your PC. If you were lucky, they would also send you drivers and other software. And if you were really lucky, all of it would actually work.

Changes and Choices

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Things have changed in the past 20 years. Thanks to evolving technology and cutthroat competition there now are many DAQ products available at very reasonable prices. And DAQ systems are more likely to work right out of the box. The drawback of so many options, however, is that you have to make decisions. And one of the most fundamental is the data communications interface, or how you get the data into your computer. Some of the more common options are PCI, USB, Web-based and wireless. Each has its own set of features that make it useful in some applications but not in others. Some of these are physical (e.g., cabling); others are functional (e.g., bandwidth).

What's Out There

PCI. These are "traditional" DAQ cards that go into a PC's card cage. The advantage of PCI-based systems is speed. You can sustain megahertz sample rates when writing to main memory. You can buy cards with onboard memories that sample up into the gigahertz range. High peak transfer rates tell only part of the story. Since the card is tied directly to the PC's high-speed bus, you can also perform tens of thousands of individual transactions per second. This can be very important when implementing test systems that have to dynamically interact with the data source of interest, as opposed to passive DAQ that just records incoming data.

Besides requiring a desktop PC, PCI's big disadvantage is that you have to run lots of cabling to the back of the PC. Unless you are acquiring data from something very close by, this can be a headache, from both physical and signal integrity standpoints.

USB. Ease of use has made USB a very popular standard for PC DAQ; you just plug it into any available USB port. These systems can offer moderately high, sustained transfer rates—I have seen systems that claim rates of a few hundred kilosamples per second. Another neat feature on a few systems is optical isolation between the USB interface and the DAQ electronics. This can help reduce noise and increase the safety of your DAQ setup.

One "gotcha" with USB-based systems is a transaction rate that can be as low as a few hundred operations/s. So while a USB-based DAQ system may be able to read 100,000 sps in continuous scanning mode, it may be able to convert only 200–300 sps when operating in software-polled mode.

Web-based DAQs. These systems let you put the DAQ hardware near the process you want to monitor, but view the resultant data in real time from anyplace with a network connection. There are two advantages. First, the proximity of DAQ hardware to the monitored process reduces analog cable lengths, and potentially results in improved signal integrity. Second, you don't have to be near the process, which can be a really good thing.

Although networks can provide high data transfer rates, what you actually get will vary wildly depending on your network hardware, who else is on your network, and what they are doing. If you work in a company where 100 MB PowerPoints regularly change hands, you may lose your ability to get real-time data. Fortunately, Web-based DAQ systems often include large local buffer memories to help cope with the vagaries of network performance.

Wireless. This newer choice has been all the buzz for the past few years; many vendors are now offering wireless DAQ. No wiring is the obvious big advantage—you can put the DAQ hardware almost anywhere you want.

The big disadvantage here is low bandwidth (below 10 sps in some cases), so these systems are generally useful for monitoring processes that don't change very quickly. Another drawback is dead spots, places where you don't get a good enough RF link to maintain reliable communications. For low-bandwidth monitoring applications in hostile environments, wireless DAQ systems may be the best choice.

The Best Choice?

So which DAQ system is right for you? The choice is a function of what you need to measure and the conditions under which you'll have to measure it. So there is no single right answer. It almost makes me wish for the bad old days when there were fewer options. NOT!

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