Possible Sources of HSDIO Acquisition Error with NI 655x Hardware: Digital I/O (DIO)>>High-Speed>>PXI-6551, Digital I/O (DIO)>>High-Speed>>PXI-6552, Digital I/O (DIO)>>High-Speed>>PCI-6551, Digital I/O (DIO)>>High-Speed>>PCI-6552
Problem: I am performing a acquisition with an NI 6551 or NI 6552 device, and I am not acquiring the data that I know I should be. Furthermore, I have a oscilloscope or digital multimeter on the lines and I notice that the logic levels are not what I expect. What is going on? Solution: Three possible problems could be causing this issue. Each of the following problems can be fixed by properly configuring your NI 655x digital waveform generator/analyzer.
Input impedance mismatch The NI 655x can be configured with a 50 ohm or 50k ohm input impedance. The 50 ohm input impedance is intended for matched-impedance systems where it is most important to keep reflections out of the line. This configuration does cause a voltage-dividing effect; the measured logic amplitude will be one-half of the source voltage. For applications where minor reflections are acceptable, the 50k ohm input impedance can be selected. Figure 1 shows the voltage divider effect of 50 ohm input impedance with a 50 ohm source impedance. Figure 1. NI 655x Input Impedance Driving a line while acquiring The NI 655x can generate and acquire at the same time. This capability allows you to acquire what you have just generated, or you can generate while another device is also driving the line. To prevent the NI 655x from driving the line, you can tristate the channels. The tristate operation effectively removes the driver circuitry from the line. The channel electronics for the NI 655x can be seen in Figure 2. Notice that the Pattern Generation and Pattern Acquisition circuitry are seperate and independent.
 Figure 2. 655x Channel Electronics To tristate the lines, use the niHSDIO Tristate Channels VI. This VI has a boolean input that you can set to TRUE to enable Tristate and FALSE to disable the Tristate. The correct usage of this VI can be seen in Figure 3. In this figure, Tristate is enabled while the niHSDIO Read Static (U32) VI is called.
 Figure 3. Tristate enable then disable Programmable Logic Levels The NI 655x allows you to program the high and low voltages that will be expected as Logic High and Logic Low respectively. This flexibility requires that you know what amplitude the signal will have when it reaches the NI 655x channel electronics. If the High or Low voltages are not configured correctly, the card does not recognize a logic high or low, and you will miss data.
The voltage ranges can be programmed in 10 mV increments from -2 V to +5.5 V for generation and acquisition sessions. An niHSDIO Property Node configures the Voltage Input High and Voltage Input Low levels as shown as Figure 4.  Figure 4. NI 655x Voltage Input High and Low Related Links: Per Cycle Tristate Programming with NI-HSDIO Testing Protection/Clamp Diodes with the NI 655x Digital Waveform Generator/Analyzer How Do I Change the Input Impedance of my NI 655x? NI 655x Attachments:
Report Date: 10/23/2007 Last Updated: 01/09/2008 Document ID: 4EMLPRVZ |
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