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Reading multiple modules and multiple channels at the same time - Help!

Setup: cFP-2000 controller connected on a back plane to two cFP-SG-140s

 

Problem: I'm writing a Labview VI that needs to read all eight channels of both SG-140s controlled by the cFP-2000 at the same time.  I currently am using the structure shown in Figure 1, with an individual FP read icon for each channel (SG-140 #2 not shown).  I did this to check the time stamp data for each channel for synchronization, but needless to say, the channels are reading at different time stamps. 

 

Would setting up Labview as in Figure 2 actually read all 8 channels at once (by configuring an "ALL" channel in MAX), or would it just seem like it because it is only getting one time stamp?

 

If so, then once the 8 channels are synched for SG-140 #1, how do I synch that to SG-140 #2?

 

(note: the hardware times have been synched to the TIme Server IP)

 

Figure 1

 

Figure 2 

 

Thanks a lot!

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First, please don't host your images on a third party website.  Not everyone may be able to access them, and there are no guarantees the files will still be there year's down the road if someone comes across your thread while trying to solve their problem.

 

What level of synchronization are you looking for?  What is your end goal for this system?

 

Fieldpoint systems are not intended to be high frequency, precise synchronization devices like a high end DAQ card is.  The update rate on the FP-SG-140 is about 1 second for all channels depending on the filter settings.  Now how far are apart each channel is read within that second, I don't know.  Your closest synchronization would be to do the All channel setup like in your figure 2, but I doubt they would all be read at the same instant in time.  In your figure 1, you actually have no control over what order the individual channels are are read.  I'm sure they aren't precisely synchronized because of that, and also each Fieldpoint read must fetch the data across the backplane.  Likewise, synchronization across two SG-140 modules is impossible because the controller can only communicate with 1 module at any given instant in time.

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With regard to the images: I didn't see an option for uploading them directly to the NI board and I don't have a public server here to load them to since access is restricted.  How do you post images for posterity? 

 

The end goal: we are running tests lasting ~5 mins. on specimens with 30+ mounted strain gauges and we need to directly compare data between those gauges at a particular time step (or I suppose in the case of tensile tests, load step).  We should really be using the RIO controllers, but we already have four cFP-2000s, eight cFP-SG-140, and four AI-110s (for load) on four backplanes, so I need to do the best with what we have.  We usually take data at 10 Hz for our tests, but since the SG-140s limit us to ~0.9 Hz, we are slowing our test down by half.  That speed is not ideal, so I'm trying to get as much accuracy and performance out of the SG-140s as possible, since they seem to be the limiting factor.  Whew. 

 

Thanks for the info about how the controller communicates.  It makes sense that it can’t read all 8 channels from one module and 8 channels from another on the same backplane at once, although I was hoping somehow it could.  Now the question is how close (or far) is the data served to the controller? 

 

I ran a test yesterday with the setup in Figure 1 (each channel has its own FP read icon in Labview) and found that the channels are generally read (though not always) sequentially 0 – 7 on module #1, then 0 – 7 on module #2, with anywhere from 0.070 secs to 0.800 secs. between each read, averaging at ~0.040 secs.  That’s probably acceptable for us considering we’ve slowed down our tests, but it does create inaccuracy.  What makes it more confusing is that the time stamp doesn’t change when the data doesn’t change, so sometimes it looks like you’re getting a time difference of 5 seconds between each channel read when that isn’t actually true.  I’ll try the ALL channel setup you mentioned today and see if there is any difference. 

 

This is probably way more information than any sane person would want, but I’ll post my excel results if anyone wants to see. 

 

Thanks!

 

Message Edited by RTPAX on 05-08-2009 06:43 AM
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For uploading images, there is a link below the edit window, (right below the Submit Post button) called Add Attachments.  Click that and it brings up a box and a browse button to browse for your file.  You can add up to 3 attachments per message.  You can upload .png and .jpg, but not .bmp (restricted because of large file size.)  You can also upload .vi's, .xls files., .zip.  Numerouse others, but there are also other restrictions like no .exe

 

As for the timing of the channels, hopefully an NI person with knowledge of the FP communication system can jump in with any insight on what kind of timing you can expect to see.

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