scan order and empty channels for SCXI-1102

[bradj0]

Hello-

I have two SCXI-1102 cards in my chassis, all measurements will be routed to ch0 of the PXI-6229 that is also in the chassis. For design reasons, I prefer to use about 2/3 of each 1102, and thus leaving about 1/3 of each 1102 empty.

Should I populate the channels in any particular order? Eg should I start at ch0 or ch31, and which direction is the scan order? e.g. 1102a/ch0 -> 1102a/ch15 and then 1102b/ch0 -> 1102b/ch15 OR say 1102a/ch15 -> 1102/ch0 and then 1102b/ch15 -> 1102b/ch0

Does this matter AT ALL with a PXI/SCXI system? I know with many DAQ devices (E-series for example) you must scan in descending order (eg. 7,6,5...0) so it makes more sense to start populating the last channel first and then leave the lower-numbers empty.



thanks-
brad

[NatasaLingurovska]

Hi Brad,

With a PXI/SCXI system it does not matter at all the order in which you populate the channels. The SCXI channels are multiplexed to the DAQ channel, so it does not matter for driver in which order it scans the channels. Which ever channel setup is most convenient for you will work just fine from software prospective.

[bradj0]

Hi-

I read through the SCXI-1102 manual and it does matter. First off, the 1102 internal MUX can only scan 1->32. If one was to scan 1, 3, 5, etc actually the MUX is going through the even channels too, but that data is discarded at some point.

http://digital.ni.com/manuals.nsf/websearch/2CDD38710DA595BB86257150006A6285
http://www.ni.com/pdf/manuals/371975d.pdf (the actual pdf)

see page 35 "Theory of Multiplexed Operation" for the specifics.

"The list can contain channels in any physical order, but the SCXI-1102/B/C  multiplexer can only sequence the channels in the order 0, 1..31. The  SCXI-1102/B/C cannot skip channels or scan channels in random order.  The ordering of scanned channels must be sequential. "

and

"Since you must scan the SCXI-1102/B/C sequentially, the driver  automatically scans channels not included in the scan list if a sequential  order is not maintained. When this happens, the maximum sample rate is  also factored into the channels that are scanned and discarded for the  purpose of completing the scan list. "


It seems that if you scanned "backwards" the max sampling rate would be something 32 times less than the max rate.


Brad

[Tom_W_[DE]]

Hi Brad-

The NI-DAQmx driver automatically reorders your scan list to avoid any delays that result from "out of order" channel scanning.  One recommendation I would have is to create your channels in numerical order and without gaps (i.e. add them as 1102a/ai0, 1102a/ai1, ..., 1102a/ai20).  This will remove the necessity of scanning "ghost" channels between two non-contiguous segments within the same module.  There are no restrictions that apply for cross-module channel orders.

So, it doesn't matter where you start or end on each individual 1102, but make sure you add the channels in a contiguous, ascending list.  Let us know if you need more info.

Thanks-

[thomson]

I would also recommend grounding any unused input channels that are in the scan list. This may improve the accuracy of your measurements.