PXI-6123 large channel count performance

[bmann2000]

I'm planning to use 9 PXI-6123 cards to simultaenously sample 72 analog signals at 100kHz, also in the chassis will be 2 Geotest GX5642DS digital differential cards sampling at 2MHz.  The total acquisition time will be just 1-second.

Each PXI-6123 has 32MB of onboard memory and has quoted performance of 500kS/s/ch with 8 channels per card, it would seems reasonable to achieve 100kS/s/ch.  

My digital and analog signals don't need to be synchronised and the GX5642DS is supplied with LV drivers.

This is the first system I put together using more than 4-cards, is there likely to be any pitfalls or performance issues I should be aware of ? 

[Ian Colman]

Hi,

You should be absolutly fine when acquiring data in the specified manner. You may run into issues if you are running continuous sampling for more than 8 seconds or so, but if you are doing a finite acquisition for about 1 second you should certainly get all of your information acquired and back to your PC. You will probably experience a delay of about a second between the acquisition finishing and the data being presented on your front panel but this is simply the time taken to return the data back from the onboard memory.

Best Regards,

Ian
Applications Engineer
National Instruments UK & Ireland

[bmann2000]

I've read PXI-6123 specifications, but I'd like more detail on how I might synchronize the measurements.  I notice there are hardware trigger lines, but had hoped I could synchronize the acquisition of all 72-channels in software.  Do you know of any NI literature that would give me an indication of the synchronisation options and the performance I could expect from each?  I imagine you can SW or HW sync, and they’d be a known performance for each option.

thanks for the quick reply to my first post. 

[Ian Colman]

Hi,

Hopefully the following resources will be of help to you:

http://digital.ni.com/public.nsf/allkb/F4AD3B87DC8CD62E862571E3007042E5
http://digital.ni.com/public.nsf/allkb/9305D5CDC61FF46286256C840065B2F4?OpenDocument
http://zone.ni.com/devzone/cda/epd/p/id/3609

Best Regards,

Ian

[bmann2000]

Thanks, quite a lot to take in.   Summary of my understanding:

The NI-6123 is an "S Series Card", The "S Series User Manual" describes the functions.

http://digital.ni.com/manuals.nsf/websearch/55C6859BE86BA5A88625724200525582

The PXI-6123 can use the "PXI Trigger Bus" but not the "10 MHz Backplane Clock" or the "PXI Star Trigger", it can use the "Real-Time System Integration Bus (RTSI)" but only up to 7-cards max, my app requires 9-cards.

When I software trigger the start of my 72-channel acquisition, each card will receive the trigger on the PXI-bus on the designated line e.g. trig line 0.  From that moment in, each PXI-6123 will be running independently using it's own onboard base-clock accurate to 0.01%.  Assuming it's 0.01% per sample, by the end of my 1-second acquisition, i'll be a maximum of 100micro-seconds out on the sync between any two channels.

This is probably okay for this application, but it seems the 6123 is good for simultaneous sampling of up to 8-channels, but not the best NI product for synchronising 72-channels.  Although it does appear to be the lowest cost option for this application.

[Ian Colman]

Yep that sums everything up quite nicely I think. You shouldn't receive more than 100us of discrepancy. The 6123 is a great for high throughput applications that require individually sampled channels so as long as you can cope with the 100us figure your application should work fine.

Best Regards,

Ian