Can I Use a 5 V/TTL DIO Module to Read 0 VDC to -24 VDC Negative Triggers?

Updated May 2, 2018

Reported In

Hardware

  • NI-9401

Issue Details

I want to read a a negative trigger from a 0 VDC to -24 VDC signal with a 5 V TTL DIO Module.
I see that the module can take up to ±30 VDC with overvoltage protection. Does this mean I can use my module to read negative triggers?

Solution

You may not read negative triggers because all levels below 0.8 V will be read as a Low state.

If you invert the signal or reverse the polarity of your measurement then you would be able to detect signal triggers according to the 5V TTL logic, where a state is LOW if the voltage level is lower than 0.8 V and HIGH for levels above 2 V.

Additional Information

Overvoltage considerations:

DIO modules have overvoltage protection. The NI-9401 for example has a ±30 VDC maximum voltage protection for a maximum of one line, while other module's protection extends to multiple lines. This means that the nominal voltage may be exceeded on a NI-9401 in one of the lines safely, but all other lines should remain at a maximum of the nominal level or the device might be damaged.

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