NI 9474 Generates Intermittent Pulses When Set to Logic Low

Updated Feb 1, 2018

Reported In

Hardware

  • NI-9474

Issue Details

I am using the NI 9474 module to generate pulses. All digital output lines are directly connected to my external circuitry or device under test. When I set logic high on DOx channel, it outputs Vsup Voltage. When I set logic low, It does not output 0V. Is this expected behavior of the module?

Solution

It is expected behavior of the NI 9474 module. When you set the output(s) on the NI 9474 to High or Low, the channel does not toggle between Vsup and COM. It toggles between Vsup and a high impedance state where the high impedance state has an unspecified leakage current. The combination of unspecified leakage current and an external circuit input impedance creates spurious pulses or intermittent pulses. Following equation will help you to understand same:

V = I (Leakage current) * R (Input impedance of an external circuit)

Refer to following article to resolve this issue Troubleshooting Digital Output and Pulse Generation NI C Series Devices

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