9421 input impedance

[bkling]

Does anybody know what the input impedance of an NI 9421 is?

Thanks,
Bryan

[TheWoost]

Hi Bryan,

We do not spec an input impedance for this module because the inputs use a current-limiting architecture, so the input impedence is dynamic based on the voltage.  You could use the <7mA max input current spec in conjunction with your expected source voltage to calculate a pseudo minimum input impedence.  Keep in mind that this is dynamic and the 7mA value is more of a guideline since it is dependent on the circuit you have attached to the input of the NI 9421.

Regards,
Andrew W
National Instruments

[Hansel]

How about a 24Vdc, 5A power supply switching on a solenoid?  So basically, a 24 Vdc, 5A standard NI desktop power supply / adapter switching 24V to a solenoid via an NI-9485 SSR and monitoring the solenoid status with an NI-9421 digital input.  In this configuration the solenoid does not turn on and the 24V is measured at 20V.  If the NI-9421 is removed from the circuit the solenoid turns on fine and the voltage is measured at 24V.   It looks like the NI-9421 module channel is drawing down the circuit.  Would making the NI-9421 input channel high impedance due the trick?  If so, how would I do that?

[santo_13]

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@Hansel wrote:

How about a 24Vdc, 5A power supply switching on a solenoid?  So basically, a 24 Vdc, 5A standard NI desktop power supply / adapter switching 24V to a solenoid via an NI-9485 SSR and monitoring the solenoid status with an NI-9421 digital input.  In this configuration the solenoid does not turn on and the 24V is measured at 20V.  If the NI-9421 is removed from the circuit the solenoid turns on fine and the voltage is measured at 24V.   It looks like the NI-9421 module channel is drawing down the circuit.  Would making the NI-9421 input channel high impedance due the trick?  If so, how would I do that?

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Please share a simplified connection diagram of the setup.