surprising increase in loop latency

[metso]

hi,

Just as it was with a former service request (see here), a LOT of time has passed and I'm still waiting for the first comment on my Labview FPGA related service request.

I reported this problem below to NI but heard nothing ever since. I only got an automatic reply to the service request saying "Occasionally, we will not be able to respond to your message within 2 business days". Now it's been 2 business WEEKS already, and still counting.

If someone has an idea or solution to this, please let me know.

I need to feed in AD sampled data simultaneously from several analog channels. I have a loop where AI1 (channel 1) data is connected to a FIFO input. If no other AI channels are visible in the loop, loop period is 250 ticks. This is shown by loop tick counter.

If channel AI0 is added, but not even connected anywhere, loop period increases to 400 ticks.

If channel AI2 is added as well, also not connecting it anywhere, loop period is 600 ticks.

Here is a screen capture of the loop with AI1 connected to the FIFO input, AI0 visible but not connected, and AI2 not even visible. This setup gives value 400 for loop2period.

As this loop is running in an FPGA it should be possible to use as many AI channels as there are without any penalty in loop timing, right? How can I apply a setup with 16 analog inputs and maximum loop period of 250 ticks (the same as with just one input channel)?

Best Regards,
mikko

[T-REX$]

Hey Mikko,

Sorry to hear that you're having Sevice Request problems with our AE department. If you were given an SR Number, I can investigate what is going on. You should be able to click on the MyNI link at the top of this page's banner to find the service request information. Also, it might be important to note what country you're from, so I know which department to contact.

At any rate, what hardware are you using? If you have a cRIO (my suspicion, as the AI nodes are outputting FXP data), what module are you using? Some modules are simultaneously sampled, like the 9215. Others are not, and need to multiplex through the different channels, like the 9205. You should check out the spec sheet for your hardware, and check if your Analog Input module has simultaneous sampling.

[metso]

hi,

Thanks for your reply! My SR number is 7402220 and my guess is the AE department handling this case is not located at where I'm from (Finland, that is).

I'm using RIO eval kit. As it uses sbRIO-9636, it's with multiplexed AI channels. I had the impression that having multiplexed channels should not affect sampling rate of each separate channel. Your comment makes me think that if only using one channel, no actual multiplexing occurs, and all samples are being fed for that single channel, whereas for more channels, sampling bandwidth is divided by the number of channels. That would explain this behavior. Please correct me if I'm wrong. All comments are welcome.

Best,

mikko