Choosing a PXIe controller for streaming 200 MBps

Question

Warning:  This is a long post with several questions.  My appologies in advance.
I am a physics professor at a small liberal-arts college, and will be replacing a very old multi-channel analyzer for doing basic gamma-ray spectroscopy.  I would like to get a complete PXI system for maximum flexability.  Hopefully this configuration could be used for a lot of other experiments such as pulsed NMR.  But the most demanding role of the equipment would be gamma-ray spectroscopy, so I'll focus on that.
For this, I will need to be measuring either the maximum height of an electrical pulse, or (more often) the integrated voltage of the pulse.  Pulses are typically 500 ns wide (at half maximum), and between roughly 2-200 mV without a preamp and up to 10V after the preamp.  With the PXI-5122 I don't think I'll need a preamp (better timing information and simpler pedagogy).  A 100 MHz sampling rate would give me at least 50 samples over the main portion of the peak, and about 300 samples over the entire range of integration.  This should be plenty if not a bit of overkill.
My main questions are related to finding a long-term solution, and keeping up with the high data rate.  I'm mostly convinced that I want the NI PXIe-5122 digitizer board, and the cheapest (8-slot) PXIe chassis.  But I don't know what controller to use, or software environment (LabView / LabWindows / homebrew C++).  This system will likely run about $15,000, which is more than my department's yearly budget.  I have special funds to accomplish this now, but I want to minimize any future expenses in maintenance and updates.
The pulses to be measured arrive at random intervals, so performance will be best when I can still measure the heights or areas of pulses arriving in short succession.  Obviously if two pulses overlap, I have to get clever and probably ignore them both.  But I want to minimize dead time - the time after one pulse arrives that I become receptive to the next one.  Dead times of less than 2 or 3 microseconds would be nice.
I can imagine two general approaches.  One is to trigger on a pulse and have about a 3 us (or longer) readout window.  There could be a little bit of pileup inspection to tell if I happen to be seeing the beginning of a second pulse after the one responsible for the trigger.  Then I probably have to wait for some kind of re-arming time of the digitizer before it's ready to trigger on another pulse.  Hopefully this time is short, 1 or 2 us.  Is it?  I don't see this in the spec sheet unless it's equivalent to minimum holdoff (2 us).  For experiments with low rates of pulses, this seems like the easiest approach.
The other possibility is to stream data to the host computer, and somehow process the data as it rolls in.  For high rate experiments, this would be a better mode of operation if the computer can keep up.  For several minutes of continuous data collection, I cannot rely on buffering the entire sample in memory.  I could stream to a RAID, but it's too expensive and I want to get feedback in real time as pulses are collected.
With this in mind, what would you recommend for a controller?  The three choices that seem most reasonable to me are getting an embedded controller running Windows (or Linux?), an embedded controller running Labview real-time OS, or a fast interface card like the PCIe8371 and a powerful desktop PC.  If all options are workable, which one would give me the lowest cost of upgrades over the next decade or so?  I like the idea of a real-time embedded controller because I believe any run-of-the-mill desktop PC (whatever IT gives us) could connect and run the user interface including data display and higher-level analysis.  Is that correct?  But I am unsure of the life-span of an embedded controller, and am a little wary of the increased cost and need for periodic updates.  How are real-time OS upgrades handled?  Are they necessary?  Real-time sounds nice and all that, but in reality I do not need to process the data stream in a real-time environment.  It's just the computer and the digitizer board (not a control system), and both should buffer data very nicely.  Is there a raw performance difference between the two OSes available for embedded controllers?
As for live processing of the streaming data, is this even possible?  I'm not thinking very precisely about this (would really have to just try and find out), but it seems like it could possibly work on a a 2 GHz dual-core system.  It would have to handle 200 MBps, but the data processing is extremely simple.  For example one thread could mark the beginnings and ends of pulses, and do simple pile-up inspection.  Another thread could integrate the pulses (no curve fitting or interpolation necessary, just simple addition) and store results in a table or list.  Naievely, I'd have not quite 20 clock cycles per sample.  It would be tight.  Maybe just getting the data into the CPU cache is prohibitively slow.  I'm not really even knowledgeable enough to make a reasonable guess.  If it were possible, I would imagine that I would need to code it in LabWindows CVI and not LabView.  That's not a big problem, but does anyone else have a good read on this?  I have experience with C/C++, and some with LabView, but not LabWindows (yet).
What are my options if this system doesn't work out?  The return policy is somewhat unfriendly, as 30 days may pass quickly as I struggle with the system while teaching full time.  I'll have some student help and eventually a few long days over the summer.  An alternative system could be built around XIA's Pixie-4 digitizer, which should mostly just work out of the box.  I prefer somewhat the NI PXI-5122 solution because it's cheaper, better performance, has much more flexability, and suffers less from vendor lock-in.  XIA's software is proprietary and very costly.  If support ends or XIA gets bought out, I could be left with yet another legacy system.  Bad.
The Pixie-4 does the peak detection and integration in hardware (FPGAs I think) so computing requirements are minimal.  But again I prefer the flexibility of the NI digitizers.  I would, however, be very interested if data from something as fast as the 5122 could be streamed into an FPGA-based DSP module.  I haven't been able to find such a module yet.  Any suggestions?
Otherwise, am I on the right track in general on this kind of system, or badly mistaken about some issue?  Just want some reassurance before taking the plunge.

Answer 1

drnikitin,
The reason you did not find the spec for the rearm time for
the 5133 is because the USB-5133 is not capable of multi-record acquisition.  The rearm time is a spec for the reference
trigger, and that trigger is used when fetching the next record.  So every time you want to do another fetch
you will have to stop and restart your task. 
To grab a lot of data increase your minimum record size.  Keep in mind that you have 4MB of on board
memory per channel. 
Since you will only be able to fetch 1 record at a time,
there really isn’t a way to use streaming. 
When you call fetch, it will transfer the amount of data you specify to
PC memory through the USB port (up to 12 MB/s for USB 2.0 – Idealy).
Topher C,
We do have a Digitizer that has onboard signal processing
(OSP), which would be quicker than performing post processing.  It is
the NI 5142
and can perform the following signal
processing functions.  It is
essentially a 5122 but with built in OSP. 
It may be a little out of your price range, but it may be worth a
look. 
For more
information on streaming take a look at these two links (if you havn’t
already). 
High-Speed
Data Streaming: Programming and Benchmarks
Streaming Options for PXI
Express
When dealing with different LabVIEW versions
it is important to note that previous versions will be compatible with new
versions; such as going from 8.0 to 8.5. 
Keep in mind that if you go too far back then LabVIEW may complain, but
you still may be able to run your VI.  If
you have a newer version going to an older version then we do have options in
LabVIEW to save your VI for older versions. 
It’s usually just 1 version back, but in LabVIEW 8.5 you can save for
LabVIEW 8.2 and 8.0.
ESD,
Here is the link
I was referring to earlier about DMA transfers.  DMA is actually done every time you call a
fetch or read function in LabVIEW or CVI (through NI-SCOPE). 
Topher C and ESD,
LabVIEW is a combination of a compiled
language and an interpreted language. 
Whenever you make a change to the block diagram LabVIEW compiles
itself.  This way when you hit run, it is
ready to execute.  During execution LabVIEW
uses the run-time engine to reference shared libraries (such as dll’s).  Take a look at this DevZone article about
how LabVIEW compiles it’s block diagram (user code). 
I hope all of this information helps!
Ryan N
National Instruments
Application Engineer
ni.com/support