How to find out max Frequency for an input?

skyfox

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PLC is a L24ERQB1B. Embedded inputs are not high speed. RPI for these inputs are currently set to 20 milliseconds and program scan time is > 1.5ms and <2ms. If I drop the RPI to 1.5 milliseconds for the inputs ensuring each scan sees a fresh update from the input card, how do I go about figuring out what the maximum number of pulses that one of these inputs can detect reliably in a given second?
 
PLC is a L24ERQB1B. Embedded inputs are not high speed. RPI for these inputs are currently set to 20 milliseconds and program scan time is > 1.5ms and <2ms. If I drop the RPI to 1.5 milliseconds for the inputs ensuring each scan sees a fresh update from the input card, how do I go about figuring out what the maximum number of pulses that one of these inputs can detect reliably in a given second?

According to theory from Nyquist, you need to sample the signal at least double the frequency of the highest signal frequency in the sample. Your data only updates every 20 milliseconds, or 50 hz. The fastest signal is half that, or 25 per second.
 
Thanks proof,

Appreciate your help. Does that mean I Can reliably sample 250Hz pulse rate if I set the RPI from 2ms to 2ms which would give me 500Hz? What determines the RPI limitations on a given module. i.e. How can I find out the fastest RPI I can use for a module in a given system.

Thanks
 
Thanks proof,

Appreciate your help. Does that mean I Can reliably sample 250Hz pulse rate if I set the RPI from 2ms to 2ms which would give me 500Hz? What determines the RPI limitations on a given module. i.e. How can I find out the fastest RPI I can use for a module in a given system.

Thanks

Don't you loose half again if your scan time is up to 2ms and RPI is also 2ms? Nyquist again. Down to 125Hz
 
Don't you loose half again if your scan time is up to 2ms and RPI is also 2ms? Nyquist again. Down to 125Hz


Dirt,


Since RPI and scan times are independent of each other, shouldn't I be able to get a fresh update from the input each SCAN (at 2ms) and therefore be able to scan a 250hz input? Or am I missing something here?
 

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