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How to connect external pullup for NI 9403 module
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patrick.li
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09-08-2021 01:59 PM
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I have a NI 9403 module that I use with the cDAQ 9171 USB chasis.
I am trying to use 8 IO pins on the NI 9403 module to read data input. However, the NI 9403 module does not have internal pullup to 5V. It only has a pulldown resistor built in. The system providing this data to the NI 9403 module is far away and the signal output is either 0V or no connection. If I add a pullup resistor I need a 5V source but the NI 9403 module does not have a +5V pin that I can use. I can use a 5V wall power supply for this 5V source but that is a more complicated setup.
Is there any way that I can get a 5V source without using external power supply? Thanks for your help!
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santo_13
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09-08-2021 03:56 PM
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Simple, use one of the DIO channels as output and write HIGH, now use this channel as a 5V source for your pullups - caution! current limited to 2mA (or the voltage drops significantly).
If I were you, I would implement this pullup at the System providing the data to 9403.
Santhosh
Soliton Technologies
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Soliton Technologies
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09-09-2021 09:15 AM
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Thanks for your reply! Yeah I was thinking using a pin as output and pull that high to provide 5V. I need to drive 8 pins but I can only spare 1 pin for this. I think I'm just going to get a 5V wall power supply. I don't understand why they don't assign a pin to give you access to 5V. The module is powered by USB so it must have 5V somewhere inside if I open the chassis up.
santo_13
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09-09-2021 11:30 AM
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It might seem like a simple thing for you, but think from a product perspective, if you accidentally connect some high voltage on that 5V pin, you're going to blow up the module along with the USB port on the computer. Moreover, this module is generic to plug into a cRIO or cDAQ and both do not necessarily connect to USB, some are standalone, some are Ethernet.
You are thinking only about your use case but as a product manufacturer, they have to think about every single use case and take a decision that would benefit the most not just a few.
Santhosh
Soliton Technologies
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Soliton Technologies
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09-09-2021 02:42 PM
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I understand now. That does make a lot of sense. Thanks for clearing that up for me! I have been wondering about the reason for quite some time.
