new chassis cDAQ-9178 has smaller buffer than the old cDAQ-9172 ?

[McFly]

Digital Waveform characteristics:

Waveform acquisition (DI) FIFO for cDAQ-9172 : 2047 samples

Waveform acquisition (DI) FIFO for cDAQ-9178 : 127 samples per slot

This means that the new HW (9178) has a much smaller buffer than the old one (9172) !

I want to execute a correlated finite generation/acquisition at 2MHz with 2000 samples. Does that mean that the new chassis will not be able to acquire the whole set of data (I will use a NI 9401 module inside the chassis)?

[RamonaL]

Hi McFly,

I just tested it with the 9178 and the 9402 (I didnt have the 9401 here), and it was working without any problems.

There are more high-performance datastreams on the new cDAQ-Chassis. So its not really easy to compare them from just one point in the specification. Well, what you want to do will work fine.

Cheers, RMathews

[McFly]

Thanks for the reply. I was not expecting that the new HW was not as good as the old one, but anyway I am grateful you have tested similar equipments.

Best Regards

McFly

[MarkGrot · Accepted solution]

The FIFO sizes are misleading and not a true indicator of the chassis streaming capabilities nor is it the only buffer.  We've done extensive testing comparing the cDAQ-9172 and newer chassis.  You won't notice the difference, especially with your application of a finite task of 2,000 samples at 2MHz.  With 2,000 samples you could run your DI task on your cDAQ-9178 chassis at 10MHz if you wanted without a problem (don't try this with continuous or long finite tasks).

The only time the FIFO size really comes into play is when you are running a continuous task at high rates, say >5MHz, though it is system dependent.  At those speeds a FIFO which is 2,000 samples deeper buys you only tens or hundreds of extra microseconds when the jitter on Windows is measured in milliseconds.

I'd be very surprised if there was a practical application that worked on a cDAQ-9172 chassis that did not work on a newer cDAQ USB chassis.