BNC

[rico1985]

Hi,

I am using NI BNC-2110 Adapter and NI USB-6251. I am wondering whether the channels in the BNC multiplex simultaneously or separately and what's the delay step (switch speed) for it to switch it from one channel to another when I aquire data from several channels. Is the switch speed a function of the sampling rat or fixed if channels don't collect data at the same time? There are time shifts among the signals from different channels in my test.

And I feel confused about how to set different sampling rates for different channels and creat local virtual channels which point to physical channels. It is said in the following link if the channels have a common divisor, I could implement your task.

 http://digital.ni.com/public.nsf/allkb/96FD2F4685065C7686256F25006EE8DE?OpenDocument

Thanks a lot. 

[Steve_B · Accepted solution]

Hi Ningyu,

The USB-6251 will use a multiplexor to sample multiple channels, which means that there will be an interchannel sample delay. The actual amount of delay will change depending on your configuration, but I have included a couple of KnowledgeBase articles that describe this delay and also have the formula to calculate your delay. The rate that the multiplexor is switching comes from the convert clock.

The article you mentioned describes a way to configure your acquisition to sample one channel more times than another channel based on the same clock. As indicated in the example you can sample channel 1 three times and channel 2 once for a single edge of your sample clock. The issue is that while you are sampling channel 1 three time, the samples are taken with a delay based on the convert clock, not a sample clock. THe image in the article shows how the samples are dispersed, so it is not identical to sampling at a faster sample clock rate based on the time between the samples.

How is the Convert (Channel) Clock Rate Determined in NI-DAQmx and Traditional NI-DAQ?
http://digital.ni.com/public.nsf/allkb/42484E84DA98053686256D32006E0494?OpenDocument

Difference Between the Sample Clock (Scan Clock) and the Convert Clock (Channel Clock)
http://digital.ni.com/public.nsf/allkb/4D1435DF82EF494186256D8A006DD6D4?OpenDocument