Limits on sample size x Sample rate

[mhtl]

I am new to Labview 7.0 and my question is actually two:

I have a PCI6024E card with a SCXI-1000 chassi and a SCXI-1100 thermocouple module and I am using the 1180 feed-through cable to access the other channels on the D/A card.

I am using Labview 7.0 and was able today to get temperature and voltage being acquired simultaneously and write a program to use the digital trigger input to trigger the acquisition which all seems to work just fine.

I need to sample at high frequency since I need to analyze the RMS voltage on a 60 hz signal. Therfore, I need about 7 kHz. I will be sampling 20 thermocouples and 6 voltage inputs for a total of 26 channels at 7 kHz which falls within the 200 kHz capability of the system. I plan to f
ilter the thermocouple signals once into Labview.

My question is:

1. Is there a limit to the sample size I can set when acquiring all 26 channels at 7 khz and how do I know what this would be? What I want to do is have an external signal trigger the acquisition every 3 seconds using the digital trigger to kick of the acquisition. That means that, let's say for 3 seconds, I would need to set a sample size of 21000 for 7 khz on each channel. Is this going to cause problems? It seems like a huge number of data points.

2. Three of my voltage signals are simply high/low 24 V signals from sensors. I was thinking of trying to reduce their voltages with a divider to 0/5 V and use the digital inputs on the data acquisition board to monitor them instead of using the analogue inputs. Is this going to gain me some bandwidth? However, I was having trouble when I tried to define a task using the global channel name control that included a digital input as well as the temperature and
voltage inputs already defined and I couldn't get it to work. Although I could simulate the digital input was working in the test panel in MAX by using a variable voltage supply into the digital input 3 on the board.

[Russell]

Hello,

1) You can actually acquire as many points as you want. You may be worried about the throughput of all this data from the DAQ board into computer memory, but I don't think this will be much of a concern for your application. You're limited only by how much memory your computer has. Mainly, you should be concerned about what you are doing with all those samples after they are acquired. Are you just using them once then throwing them away? (Such as the "Cont Acq&Graph Voltage-Int Clk.vi" shipping example.) Or are you streaming them to disk? If you just keep them all in computer memory, then there would be some maximum number of samples you could hold but that number would be system dependant and you'd just have to experiment to find that number.

2
) That's a great idea to take those voltage signals and make them digital inputs. This way you'll also free up some more bandwidth giving you the option to increase the sample rate on your other channels. However, you won't be able to include any DIO channels in the same task with your other analog channels. You need to create a separate task for these digital input channels. Lines 0,1,2, and 4 will be reserved for communication with your SCXI chassis by the way. You also won't be able to do any buffered sampling like you can with the analog channels. The DIO for the PCI-6024E is only immediate and software-timed. (You must poll the status of the line with a software loop.)

Let me know if you have any other questions.

Russell
Applications Engineer
National Instruments
http://www.ni.com/support