There’s an old story about a backwoods sign that reads, “Pick your rut carefully. You’ll be in it for the next 60 miles.” The mental picture is enough to make us chuckle—someone standing by a muddy sign, holding his hat, scratching his head, and trying to decide just which rut is best.
The same can happen in system design. When someone says a certain word, we categorize it into one of our own mental ruts. A preset impression might be that PXI means small and inexpensive or VXI is expensive but fast. Rack and stack could conjure an image of big and slow, but capable. Of course, none of these impressions is totally true.
We all look for ways to improve tests and develop them faster at a lower cost. It’s in our best interest to consider all the alternatives and not get entrenched in a single architecture.
For example, assume you want to automate a small test with a PC using software you already have. Let’s just make a 5½-digit voltage measurement on a single channel, about the minimum you could expect from any system. If you include the cost of I/O and cabling, the entry cost for this simple system in VXI is about $6,800, approximately $3,500 for PXI, and roughly $1,200 for rack and stack.
The obvious conclusion is that VXI is more expensive than PXI. But, we often overlook the cost of rack and stack, which is much less than either card-cage format. This is somewhat surprising considering that each stand-alone instrument has its own front panel and intelligence to give it autonomy—essentially a mini-system unto itself.
Why the cost discrepancy? Primarily, it is because rack-and-stack products outsell card cages by a wide margin. This creates manufacturing economies of scale that tilt heavily in favor of the stand-alone instruments.
Card cages begin to show their advantage as systems get larger and channel count increases. As the system grows even more, the cost of VXI can approach or even be less than that of PXI.
You’ll rarely see a system that uses one type of architecture throughout. There’s a reason for this. PXI, VXI, and rack and stack have distinct advantages, and it’s up to the system designer to extract the most speed, accuracy, throughput, repeatability, and reliability from the system while minimizing design time and cost.
Not all instrument functions are available in every form factor, so system hybrids are commonplace. Fortunately, standards like IVI component drivers and standard PC I/O are making the technology marriages easier—not simple, but easier. This move to open standards will dramatically improve system development time and lower the cost of test.
Standards may not fill in all the ruts, but at least they can somewhat level out most of them. It’s up to the system designer to carefully look at the map and consider all available routes.
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Published by EE-Evaluation Engineering
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September 2003