Why Does My USB-6501 Not Output a 5 V "On" voltage?

Updated Jan 5, 2018

Reported In

Hardware

  • USB-6501

Driver

  • NI-DAQmx

Issue Details

When I output a digital signal on my USB-6501, the "on" voltage is less than 5V. Why is this?

Solution

The USB-6501 gets its voltage from the USB which causes the digital output voltage to be dependent on the bus voltage. This may vary from one motherboard manufacturer to another, but the USB 2.0 specifications state that the valid range is from 4.4-5.25 VDC.

You can incorporate an external resistor between the 5 V bus and the digital output line that will allow the digital output line to be pulled up to the 5 V rail. This user supplied resistor allows the USB-6501 to drive up to 8.5 mA of current to a load. As seen in the diagram below, the external resistor is in parallel with the 4.7k on-board resistor. 

Additional Information

Instructions on how to select the appropriate value for the external resistor is included in the NI USB-6501 User Guide under the Digital I/O section. 

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