Re: Fwd: Re: NI SCXI and E series boards

On Mon, 14 Oct 2002, Caleb Tennis wrote:

>
> Calin,
>
> We use an SCXI chassis on our comedi daq systems with a lot of success.  I
>  use the patch that Brad Keryan wrote for comedi 0.7.63 with some
>  modifications to make it work with our own system.  I'd like to see it
>  integrated into comedi, but in it's current state Dave didn't like how it
>  was built (and I agree with him) and we never thought of a much better way
>  of doing it.
>
> Aside from that, I think of the modules as signal conditioning multiplexers.
> We use them for thermocouple and pressure transducer measurements.  It
> basically turns 4 of the DIO lines into serial data channels to communicate
> with the SCXI box, and you are able to select which channel you want to
> measure.  It basically puts the signal you select onto Analog Channel 0 and
> then you measure it using a comedi_data_read.  That's strictly for analog in;
> my experience is quite a bit less for analog out and anything digital, though
> I assume they work very similiarly.
>
> If you want to use linux, and need high channel counts or really good signal
> conditioning, it's the way to go (in my opinion).  Especially if your budget
> isn't all that tight - these modules can get quite pricy.
>
> Caleb


Caleb,

(Thanks for replying) Ok, so the SCXI 'protocol' is well documented
and getting the modules to do what I want shouldn't be too much
trouble from comedi, correct?  I just have to do some magic on the DIO
lines...

Now on to my next question: Multiplexing.  What I understand is that with
some of the modules, the idea is that you have something like 32 channels
coming in to 1 channel on your DAQ board.  You then have to demultiplex
these channels in software.  Is that correct?  How is synchronization
between the DAQ device's sampling of the 1 channel and the SCXI hardware's
multiplexer guaranteed?  The timer on the NI board? Is it possible to use
this system for control, and not just measurement?

-Calin

Received on 2002-10-14Z15:24:12