Physics 3100 Electronics, Fall 2008, Digital Circuits 1 LabView Exercises: Part III The working VIs should be handed in to the TA at the end of the lab. This is a lab under development so we may experience some glitches the first time through. Please bear with us. Initializing the NI 6251 and SCC68s The NI 6251 Multifunction I/O allows rapid analog to digital conversion and input to the computer as well as rapid Digital to Analog conversion and output from the computer via the USB port. It also provides digital I/O capabilities as well as some timer functions. We have interfaced this device to an SCC68 box containing analog and digital buffers to protect the sensitive inputs and outputs of the 6251. These two devices are connected by a 37 pin cable. The banana plugs and BNC connectors which we have added to the SCC68 provide a convenient interface to the student s experiments. Exercise 1: Using MAX to initialize the NI 6251 The first time this hardware is used on any given computer it must be initialized. The Measurement and Automation Explorer (MAX) software provides an easy way to identify the NI hardware connected to the computer. Follow the following steps: 1) Once you are sure that the NI 6251 and SCC68 are correctly connected and powered on, launch MAX by double clicking on its icon on the desk-top.
Physics 3100 Electronics, Fall 2008, Digital Circuits 2 2) On the left side of the window which MAX opens you will see a column entitled Configuration. Double-click on Devices and Interfaces to expand the sub-category (or click on the + box). Doubleclick on NI-DAQmx Devices. The expanded sub-category should show an NI USB-6251 (Mass Termination): Dev xx (there will only be one such entry for your computer). 3) Click on the NI USB 6251 entry (it should be showing a green circuit board on the left to indicate that it is connected). 4) Click on the Properties tab at the centre top of the window. This will open a Device Properties dialog box. On the Accessory tab which is displayed there will be a check-list. Scroll down the list to find SCC-68 as shown. Click on SCC-68 and on OK.
Physics 3100 Electronics, Fall 2008, Digital Circuits 3 5) Now click on the tab Test Panels.. at the top of the window to display the test panel dialog box. This panel will allow you to test the functioning of your NI 6251 and the SCC 68 which is connected
Physics 3100 Electronics, Fall 2008, Digital Circuits 4 to it. 6) Begin by testing the Analog Input channels (the first tab). The channels which are connected via the SCC 68 are Dev*/ai4 and Dev*/ai12. First select Channel Name Dev*/ai4 and set up the test for Continuous Mode, and the RSE Input Configuration, as shown below. If there is no signal source connected to the BNC connector labelled AI0 on the SCC 68 the display chart should show noise similar to that displayed on the figure when you press Start. 6) Connect the output from your function generator to the input AI0 BNC connector. Since the acquisition rate specified in the test panel is 1000 Hz (see figure above) your function generator frequency should be set to a frequency which is less than 500 Hz. (You must sample at least twice as fast as the highest frequency of interest..this is the sampling theorem). Choose a suitably low frequency. Also, the input should have an amplitude less than 10 volts. If the output displayed on the panel is as you d expect, click on STOP and perform the same test on Channel Name Dev*/ai12 using the same set up parameters. If both channels pass this test, move on to the Analog Output test by clicking on the Analog Output tab at the top of the window. 7) Connect the Dev*/ao0 output from BNC AO0 on the SCC 68 to your oscilloscope. Set up channel 0 as shown below
Physics 3100 Electronics, Fall 2008, Digital Circuits 5 You should see a sine wave on your oscilloscope. Change the amplitude and frequency to verify that everything is working correctly then repeat the test using Dev*/ao0 and the AO1 BNC connector. 8) Finally, click on the Digital I/O tab at the top of the window and test the 4 output and 4 input digital lines. These are connected to the coloured banana jacks and are labelled DO0 through DO3 and DI0 through DI3. Set up the test panel as shown below, using port1 and click on Start. Lines 0 through 3 are assigned as inputs while those from 4 through 7 are outputs. All 8 lines are connected internally to pull-up resistors so the 4 input lines )bits 0 through 3) show up as high unless something pulls them down. Connect any of the 4 lines DI0 through DI3 to ground using a wire with banana plugs and the corresponding circular green light on the test panel should go dark.. Next, connect one of the output lines DO0 through DO3 to an LED (use a 1 kω series resistor). Then using the test panel s slide switches you should be able to turn the LED on and off. This demonstrates that the digital I/O is working. After you have completed these tests you may continue with the next exercise.
Physics 3100 Electronics, Fall 2008, Digital Circuits 6 Exercise 2: Data Acquisition Using the NI USB-6251 plus SCC 68 The National Instruments USB-6251 provides 16 single-ended or 8 differential channels of analog input. It does this using a single 16-bit analog to digital converter capable of digitizing at a maximum rate of 1.25 MHz. The 16 channels are provided by multiplexing the input to this A/D converter. If more than one channel is used the maximum aggregate sampling rate is 1.0 MHz (i.e., 500 khz per channel for 2 channels, etc). It also provides two separate 16 bit analog output channels which can operate at up to 2.86 MHz for a single channel or 2.0 MHz aggregate for two channels. See the attached data sheet for detailed specifications. The NI SCC 68 provides the interface between the NI 6251 and the user s experiment. We have enabled two analog input channels, channel ai4 and ai12 each of which is buffered by an Analog Devices OP249 op-amp to provide operation over the full range of specifications of the NI 6251. The two digital outputs are also buffered by OP249 s operating with a series 51 Ω resistor to protect against short-circuits on the outputs. Finally, we provide 4 digital input lines (port 1, bits 0 to 3) and 4 digital output lines (port 1, bits 4 to 7) which are buffered by a 74LS244. There is a 560 Ω pull-up resistor on each digital input and output. The NI USB 6251/SCC 68 combination is a flexible and powerful data acquisition and control system and the exercise described below is an introduction to using such a system.
Physics 3100 Electronics, Fall 2008, Digital Circuits 7 Exercise 2: Data Acquisition using the A/D Converter In this exercise you will build a LabView VI for acquiring data from the two analog input channels ai4 and ai12. The VI allows you to change the sampling rate and to save the data to an.lvf file. (This is LabView s text formatted data file format.) Launch LabView and, under New, click on Blank VI. Decorate your front panel as shown below. The corresponding block diagram should be as shown in the next figure.
Physics 3100 Electronics, Fall 2008, Digital Circuits 8 You will notice that the block diagram contains three loop structures: the outermost loop is a while loop which terminates only when the STOP button is pushed, the large inner loop is a Case structure which executes every time you press the READY button. The small inner loop executes when you push the Save? button and saves the data to a file. When you place the DAQ Assistant on the block diagram you will need to configure the physical channel being used (dev*/ai4). The configuration dialog box should open when you place it on the block diagram. If it doesn t, right click on the icon and, from the menu displayed, select Properties. Set up channel dev*/ai4 as shown.
Physics 3100 Electronics, Fall 2008, Digital Circuits 9 This VI allows you to collect # of samples data points at Sampling Rate. If you connect a sine wave to the AI0 input channel. The sampled data is displayed on the front panel graph. If you like the look of the data you can save it by pressing Save?. Save a set of data and open the.lvm file with Windows NotePad to see the organization of the data file. Once your VI is working, save it to give to the TA. Then, click on Save As and choose Substitute Copy for Original and give it a new name so that you can modify the VI for the next exercise.
Physics 3100 Electronics, Fall 2008, Digital Circuits 10 Exercise 3: Generating a Square Wave Output using the D/A Converter If you modify the VI as shown by the Front Panel and Block Diagrams below, you will be able to generate and output a waveform which you can use to excite some external circuit. The A/D converter can then be use to view the excitation and the response.
Physics 3100 Electronics, Fall 2008, Digital Circuits 11 As an example we suggest using a square wave excitation of an RLC circuit. Set up the D/A converter to provide a 50 Hz square wave, amplitude of 4 v. using a size of 1024 samples. This output should be configured to appear at Physical Channel Dev*/ao0. Build an RLC resonant circuit as shown Use the output of the D/A (from BNC AO0) as V i. Connect AI0 to the input side of the RLC circuit and connect AI1 to the output side of the circuit. When you click on the READY button, the VI generates roughly a single cycle of the square wave which results in a 4 volt step at the input to the resonance circuit. The circuit then rings at its characteristic frequency ω 0 = 1/ LC. The ringing decays rapidly with time because of the internal resistance of the wire used to make L1. Measure the frequency of the ringing and estimate the decay time. Save the VI and give the results to the TA.
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Specifications listed below are typical at 25 C unless otherwise noted. Refer to the M Series User Manual for more information about NI 625x devices.