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Configure NI Digital IO Device for a 3.3V TTL Digital Output

Updated Mar 13, 2019

Reported In

Hardware

  • USB-6509
  • Digital I/O Device
  • PXI-6509

Driver

  • NI-DAQmx

Issue Details

  • I have a National Instruments DAQ product that supports digital operations at 5 V TTL logic levels. I need to drive 3.3 V TTL. Is there anything that I can do?
  • Can I programmatically change the digital output of PXI 6509 from 5V to 3.3V TTL?

Solution

Almost all of National Instruments digital DAQ products support only 5 V TTL logic, as this is the most prevalent. If you need to read 3.3 V signals as an input, you may generally directly connect the signals to the DAQ board because 5 V TTL considers anything above 2.2 V to be a logical high. However, if you are outputting signals to a device that requires 3.3 V signals, you will need to reduce the voltage. The best way to do this is to use a bus transceiver with voltage shifting, such as these:
     
    These can also be used for voltage shifting on input, should it be necessary.

    Additional Information

    There are 3.3 V devices that are 5 V tolerant. For those devices, you can directly connect the digital output without any bus transceiver. Also, both NI R Series and NI HSDIO Series products support 3.3 V natively.

    The PXI form factor for the NI 6509 directly pulls the 5V from the VCC on the chassis backplane. This VCC is not user selectable because of which, the PXI 6509 can only output 5V signals.

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