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Changing the Default Power-On State of the Digital Lines on the NI 435x

Hardware: Modular Instruments>>High-Precision DAQ (Dataloggers)>>DAQCard-4350, Modular Instruments>>High-Precision DAQ (Dataloggers)>>DAQPad-4350 for USB, Modular Instruments>>High-Precision DAQ (Dataloggers)>>DAQPad-4352 for USB, Modular Instruments>>High-Precision DAQ (Dataloggers)>>PXI-4351, Modular Instruments>>High-Precision DAQ (Dataloggers)>>DAQPad-4351 for IEEE 1394, Modular Instruments>>High-Precision DAQ (Dataloggers)>>PCI-4351

Problem: I want to change the default power-on state of the NI 435x device to be logic low. What is the input impedance of the digital lines, and how do I change the default power-on state of these digital lines?

Solution: The default power-on state of the NI 435x devices is logic high, and you can change this default setting by using a pull-up resistor between 10 kOhm and 100 kOhm.

To calculate the most appropriate value for the pull-down resistor, assume the worst-case scenario. That is, assume that the pull-up resistor is 10 kOhm, and calculate an initial state pull-up value of 0.4 V. In the calculation below, use 0.4 V instead of 0.8 V to provide extra room in case of noise.

Also, assume the worst tolerance allowed by the PXI or PCI specifications. Assume the tolerance that occurs in the 5 V voltage level that is defined by the PCI specification, which is more permissive than the PXI specification. The tolerance for the 5 V voltage level under the PCI specification is +/- 5%, so the maximum voltage present in that line is 5.25 V.

Assuming that you connect the pull-down resistance from the digital line to ground, you have an equivalent circuit that consists of a voltage source (VIN = 5.25 V) in series with the pull-up resistor (R1 = 10k) and with the pull-down resistor you provide (R2). You want the initial voltage on R2 to be VR2 = 0.4 V, so the value of R2 under these conditions is:
R2 = [ R1 * VR2 / ( VIN - VR2) ] = 10 kOhms * [ 0.4 V / ( 5.25 V - 0.4 V) ] = 10 k * 0.082 = 824.74 Ohms

Therefore, a pull-down resistor around 825 Ohms is sufficient to change the default power-on state of the NI 435x to logic low.

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Report Date: 07/16/2004
Last Updated: 08/23/2004
Document ID: 3BFF33Q8

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