PXI-6255 Multiplexer

[jilwan]

Hi Together,

i`m using a PXI-6255 / 80 Channels / 1,25MS/s... I would like to know the settling-time t for the mux between 2 channels.

Is it always the same? if i jumped from channel 1 to 10, is it  10*t or have another equation??

My second question: i`m reading 1000 samples à 5kHz on 50 channels

Does it mean 1 Sample / Channel per multiplexing or??

Many thanks in advance

[John_P1]

Hi jilwan,

Here are the numbers from the 625x Specifications:

                

Jumping from channel 1 to 10 should be the same spec as jumping from channel 1 to 2 (the channels do not have to be multiplexed in any particular order).  Configuring a task to have multiple channels in it will scan each channel one time before proceeding to the next.  Since your rate is 5 kHz, this gives a 200 us window for all samples to occur.  Since there are 50 channels, the multiplexer would switch every 4 us.  Looking at the chart above, 4 us should be acceptable settling time for source impedances under about 1k.

If desired, it is possible (although not exactly straightforward) to configure the DAQ card to sample each channel multiple times before switching to the next (e.g. 1000x channel0, 1000x channel1, etc.).  Here is an example showing a way to do this:

Read Multiple Channels Sequentially Using DAQmx

Best Regards,

[jilwan]

Many thanks you for the answers,

i would like to ask about the FIFO on the PXI-6255:

in the Spezification they`re saying that the size is only 4095 S:

If i`m using the 80 Channels à 5kHz and reading 1000S, it means that i have a block (Data-package) from 80*1000S, does it mean that the block will be formed on the controller? How can the controller (or my read Block in Labview) be sure that the Values (Samples) have not been overwritten by others?

Regards

[WilliamEarle]

When an acquisition is running, data on the device is automatically flushed from the AI FIFO on the device to the host (be it an PXI RT controller, Windows machine, etc.). On the host, DAQmx allocates a buffer in memory to hold samples from the acquisition. The size depends on your task configuration.

The data transfer from the device to the host can usually happen much faster then the device can fill up the AI FIFO. However, if many devices attempt to use the PXI bus at once, it is possible for the FIFO to fill up. In this case, DAQmx would return an error informing you the FIFO overflowed.

I also am assuming your task is a finite acquisition. For a continuous acquisition, if you acquire more samples than the host buffer can hold without calling read, DAQmx by default will overwrite the oldest samples and return an error when attempting to read the overwritten samples.