Unexpected Input Readings on PCIe 7822R

Updated May 31, 2018

Reported In

Hardware

  • PCIe-7822

Software

  • LabVIEW FPGA Module

Issue Details

I have the PCIe 7822 R Series Digital I/O module and am performing digital input readings with the device in LabVIEW FPGA. My digital input signal is not reading as expected. What are some possible causes of this?

Solution

There are a variety of reasons for improper digital readings with the 7822R:
  • Improper physical connections/wiring. The front connectors on the 7822R are 4 68-pin VHDCI connectors. All the DIO channels on Connectors 0 through 3 are routed with a 50 Ω characteristic trace 
    impedance. Route all external circuitry with a similar impedance to ensure best signal quality. It is recommended to use the SHC68-C68-RDIO2 Digital Cable for R Series which then connects to the SCB-68 HSDIO Shielded Connector Block for R Series and HSDIO Devices.
  • Digital input lines set in the wrong state. When powered on, the DIO channels on the device are set input low with pull-down resistors. The input impedance is set to 50kΩ. Ensure your LabVIEW FPGA code is not programmaticly setting your input pins to be in a low impedance state when they should be in a high impedance state.  
  • Signal source not outputting expected waveform. You may want to consider probing your signal source with an oscilloscope to ensure the digital signal you are trying to read is as expected. Ensure your 7822R is configured for the appropriate digital input logic level so digital highs and lows are properly registered.
  • Improper code configuration. As a starting point, it is recommended to use one of the digital input examples that ship with the R Series drivers which can be found in LabVIEW by navigating to Help > Find Examples > Hardware Input and Output > R Series > Basic IO.

Additional Information

To ensure the DIO pins you wish to use for input on your 7822R are in a high impedance state, you can set the Set Output Enable property to false for each of your channels you want configured for digital input. You can then use a ohmmeter to probe each pin and ensure the input impedance is indeed 50kΩ.

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