1010 chassis, PXI-6052E; accessing unused AI channels

[ColeTrain]

Hello Chris,

I went and dug into this issue for you a bit and here's what I have found.

1. The SCXI will always have to get its analog data back to the computer through an analog line.  So there will always be an analog input line reserved to do this.  By default this is AI0.  I know the knowledgeBase is a little confusing, and I'm working on clearing it up, but in the first sentence of the solution it reads: "Just like when using a PCI-based E- or M-Series card to control a stand alone SCXI chassis, the PXI-based card's Analog Input Channel 0 is used to collect the multiplexed data from the SCXI portion of the chassis."

2. The channels are scanned in the order you input them.  So if you enter SCXI-AI0, DAQ Card AI1, and then SCXI-AI1, this will be the channel order as they are scanned.  Both the DAQ card and the SCXI module will hav its own multiplexer and DAQmx will route the sample clock to each portion in its own order.  

3. I believe that only the channels that you configure are scanned, but I didn't see anything definitive one way or the other on this.  

Hope this helps!

[chassan]

Hi Cole,

Thanks for the information.

Can you please post a link to the knowledge base entry that you referenced in your previous post?  The information that I got came from the "DAQ Hardware Overview Guide" (October 2000) that I downloaded from ni.com (http://www.ni.com/pdf/manuals/370097d.pdf).  On page 16-2 it states "If you use the internal connection to the SCXI bus instead of cabling to an SCXI module, all the regular analog input channels on your device are available for other uses."  Maybe the phrase "regular analog input channels" is defined as all of the ai channels except for ai0?

Thanks,

Chris 

[ColeTrain]

Hello Chris,

I think the problem here is outdated material and getting all of our facts clear.  This KB talks about the ability to use all lines with tDAQ, but it is unclear if this is with only tDAQ or also DAQmx.  The other KB I was referring to was the very first one that Jim pointed you to.  I can check and get all fo the facts strait, but out of curiosity, have you just tried to do any of this?

I don't have this hardware set up, but I can normally learn a lot from just trying things out.  

[chassan]

Hi ColeR,

"...have you just tried to do any of this" is indeed a valid question.  Unfortunately, I cannot afford the time to "experiment" to determine how the pieces of hardware work together.  This would require disassembling testers, reallocating resources, experimenting, reassembling testers, and re-qualifying testers.  

However, since the chassis, controller, cards, and software (LabVIEW) were all designed by NI to presumably work together, it seems reasonable to believe that there is someone at NI who knows how these things work together.

Best Regards,

Chris

[Justin_M]

Hi Chris,

Cole brought me up to speed on your question(s).  It looks like that KB has led to a lot of confusion about how the system actually works.  Let me try to clear some of that up.

When you use a combo chassis, the DAQ device in the right most slot (either E or M series) uses an internal channel to scan the AI data coming in from the SCXI backplane.  This internal channel is multiplexed in with all of the other channels on the front connector.  This means that NONE of the front connector AI channels are reserved by SCXI.  You can use any of the AI channels through the front connector, as long as you keep in mind that there is only one AI timing engine on the DAQ device (meaning all AI channels, DAQ or SCXI, must be in the same task, running at the same rate).

Now for your previous questions:

1. What is the channel order in each scan?  For instance, are all of the scxi channels scanned first and then the "regular analog input" channels?
2. Are the channels scanned in consecutive ascending order?
3. Are only the configured channels scanned, or are all of the channels scanned (I read something somewhere about the scxi channels all being scanned consecutively in ascending order whether or not they are actually configured)?

As Cole mentioned, the channels are scanned in the order of your scanlist (with one exception, which you have already mentined in your question).  Assuming you are only using SCXI modules that support "random scanning," all DAQ and SCXI channels can be scanned in any order.

The one exception involves SCXI modules that do not support random scanning.  These modules wil always start scanning on a particular start channel, and can only scan in consecutive ascending order from there.  For example, if you want to scan channels 2 and 5 on a module that doesn't support random scanning, the module will actually scan channels 2, 3, 4, and 5 (taking 4 convert clock pulses) and return only 2 and 5 to the user.  Off the top of my head, I know that all of the 15xx modules support random scanning, and so do the 1125 and 1112 modules.  A big one that does NOT support random scanning is the 1102 module.  You can check the manual for a specific module if you are not sure about the random scanning support.

I'll work with Cole to get that KB updated, and hopefully clear this up for future users.  Let us know if you have any more questions.

Thanks,
Justin M.
National Instruments

[lab_boy]

Hello Justin,

This information was helpful but correct me if I am wrong. From what you said if you use DAQ card in combo chassis the SCXI part is controled by internal channels hence you have all the AI lines on the DAQ card available, and hence you can use then for data acquisition.

Cheers

-Lab

[Justin_M]

Hi Lab,

Yes, you are correct.  For example, let's say you have an 8 channel MIO DAQ device (Dev1) in the rightmost slot of a combo chassis.  The DAQ device MUX is connected to channels ai0-ai7 AND the SCXI backplane channel, we'll call aiSCXI.  You can create the following DAQmx channel list:

[Dev1/ai0, Dev1/ai4, SC1Mod1/ai2, SC1Mod4/ai27, Dev1/ai2]

and the DAQ device would MUX through the channels:

[ai0, ai4, aiSCXI, aiSCXI, ai2]

None of those ai0-ai7 channels are used to input the SCXI channel signals.

-Justin

[chassan · Accepted solution]

Justin M,

Thanks for the great information!  This is exactly what I needed to know.

Best Regards,

Chris