Getting Small. Does one modular standard beat the others?
Larry Desjardin- May 14, 2012
In my last blog post I summarized the advantages of downsized instrumentation. More portable, more flexible, higher performance, and lower overall cost were all advantages depending on the application.
It’s pretty clear that modular instrumentation delivers significant space savings over traditional instruments through the elimination of keyboards, displays, and redundant power supplies. But let’s look specifically at the modular systems themselves. There are three open modular standards today: VXI, PXI, and AXIe. Does one clearly deliver greater size reduction than the others? The answer: it depends. Let’s look at the theoretical advantages of each.
PXI is the smallest of all module sizes. If each of your functions can fit onto a single module, then there is a good chance that PXI will have the smallest footprint of the three architectures. But if they can’t, you will need to go to multiple slots. Do you save total space then? This brings us to another factor- what size are we talking about reducing?
Typically (but not always) rack height is the key criteria. Essentially, if a chassis can fit into the depth of a rack, then the key criteria is how little rack height the instrumentation requires, and the depth is irrelevant. In this case VXI may have an advantage, because it has the greatest module depth of the three architectures. This gives it a theoretical advantage of how much instrumentation volume (cubic inches of instruments) can exist behind each inch of rack height. For packing up circuitry, VXI may be the one with the advantage.
But all those instruments need to be powered. While Moore’s law has decreased the size of silicon chips, it has also increased the power dissipation density. Here PXI typically supports 30 watts, VXI about 100 Watts, and AXIe 200 Watts. Chassis have been designed in each architecture that exceed these numbers, but they are a good relative rule of thumb. In this case, AXIe has the highest power density when normalized per rack-inch. It could have a key size advantage for high power instrumentation.
To summarize the three paragraphs above: PXI delivers the most slots per rack-inch, VXI delivers the most instrument volume per rack-inch, and AXIe delivers the most watts per rack-inch. Depending on your key constraint, each of the architectures may have a technical density advantage.
Then there’s another aspect: what if you don’t fill all the slots in a chassis? The absolute chassis height may be the critical spec. My favorite comparison is to think in terms of 4U rack height. (1U = 1-3/4 inches, so 4U = 7 inches). Why 4U? Because you can find 4U modular chassis in each of these three architectures. 4U will give you 17 PXI instrument slots (20 with some restrictions), 5 AXIe instrument slots, or 4 VXI instrument slots. Does your application fit completely into one of these combinations? If so, you may have found your answer. AXIe also offers 2U 2-instrument slot chassis, which can be used for smaller applications or to augment another chassis.
For the bench, the dynamics change completely. Instead of rack-inch, the footprint of the system is often the most important. Also, a full rack-width chassis is no longer the optimal design like it is for the rack, allowing many more options. Here the short depth of PXI can be a major advantage. There are many more creative packaging solutions as well. I’ve seen VXI and AXIe systems standing “vertically” on one side to decrease the footprint on the bench. After all, there are no keyboards or displays that become rotated when a modular chassis is pivoted on its side!
All of these factors are considered when vendors choose a platform for a product. You don’t have to be steeped in the theory, this summary was meant to be just a rough overview of some of the trade-offs. Pragmatism dictates penciling out the system in each of the architectures to see what the actual size implications are. That’s the first big step to getting small.