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NI 9421 differences in reading frequency or digital input?

I am trying to read the frequency of a flow meter using NI921 in DAQ-9178 chasis. 


Sounds like I need to choose Acquire signals-> counter input->Frequency instead of Acquire signals->Digital input->line input when I choose physical channels in DAQ assistant configuration.


What's interesting to me is that I only see four channels if I choose to read "Frequency", and I can see 8 channels when I choose "line-input". The NI 9421 have 8 physical channels. I wonder why I can only see 4 of them (0~3) in "Frequency", and how can I get the other 4, what's the difference between line input and frequency?





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Have you read the manuals? Does not sound like it. The digital lines are with the 9421. The counters are in the chassis. There are only 4. You can't access what does not exist.
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Thanks. The manual is not easy to understand. I though 9421 can do the counting by itself with a internal clock provided by the chasis. If the counter is provided by the chasis, how does these counters were assigned to the physical channels in the module, where the inputs and outputs are assigned or collected? Say, if I want to count the frequency of my flowmeter, I physically connected the relay output of the flowmeter to one of chanels of 9421, and trying to read the frequency of on/off. If the channel I selected were the counter in the chasis, how do I know which 9421 channel the counter is assigned? How does it work?

Another question is how many count jobs I can do simultaneously? For example, how many flowmeters I can use?  

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Message 3 of 5

Hi hrwang,


You can try looking at some of our Community Examples to get a better understanding of how this will be set up in code. Here is a Digital Frequency Counter example.



Applications Engineer
National Instruments
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The counters are provided by the chassis, and inputs/outputs to those counters can be mapped through digital modules or (on the 9178 and 9188) the chassis PFI terminals.


The DAQmx Device Terminals help will indicate the default line mappings for the four counters when used with a module. You can get to it by either:

  • right-clicking the NI 9421 in MAX and selecting 'Device Pinouts', or
  • opening the DAQmx Help (in the start menu) and going to NI-DAQmx Help->NI-DAQmx Device Terminals Help->DAQ Devices->NI 9421

These are the defaults. You can also override these terminals by using channel properties; as an example, if you're doing a frequency measurement on cDAQ1Mod1/ctr0 (which by default will use /cDAQ1Mod1/PFI0 as your input terminal), but you want to do the count on pin 2, you can set the CI.Freq.Term channel property to /cDAQ1Mod1/PFI2.

Brandon Streiff ·
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