Technical Discussions on any InterfaceKits


Postby Chav » Mon Mar 09, 2009 11:46 am

Since our application requires reading 25VDC signals and sinking 25VDC outputs the 0/16/16 interface kit seems like the perfect product. The only issue is the speed at which the digital inputs update.
We are attempting to read two 0-72hz signals. The 0/16/16 interfacekit's spec says that the digital input update rate is 125hz, but we have noticed that it starts to miss state changes above 60hz. Is our understanding of the digital input update rate incorrect or is there something we can do to increase this rate?

Thank you for your time.
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Re: 0/16/16

Postby Patrick » Mon Mar 09, 2009 11:58 am

It should be able to detect about 60Hz as long as your pulses are at least 3ms wide. It won't be able to detect 72Hz because 2 updates = high/low = 2 updates/Hz = max 125/2 = 62.5 Hz detection.



Re: 0/16/16

Postby Chav » Mon Mar 09, 2009 12:16 pm

Thank you for the quick reply. One solution I thought of was to use a "clock divide" circuit to latch the inputs and half their frequency. We don't need the quarter angle accuracy. Is there another, more elegant, solution?

Thanks again.


Re: 0/16/16

Postby Chav » Wed Mar 11, 2009 6:32 pm

Now that the frequency is divided by 2 we are now realizing that the event handler is reporting the events out of order.

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Re: 0/16/16

Postby fitchett » Thu Mar 12, 2009 8:45 pm

?? What do you mean?



Re: 0/16/16

Postby Chav » Fri Mar 13, 2009 10:51 am

I was wrong about the events coming in out of order.

To solve the initial problem I used D flip flops to half the input frequency. Now the maximum input frequency is 36Hz.

The current problem:
A closer look shows that the interface kit is occasionally missing changes of state from a 36 Hz square wave input. Once a high to low transition is missed the following low to high transition is ignored because it appears that the input has never changed state.

To avoid aliasing the sample rate needs to be twice the signal frequency. So if we have a 36Hz input, the sample rate needs to be at least 72Hz. Which is what I believe Patrick was saying.

Since 72Hz is well bellow 125Hz I'm confused as to why the interface kit is still occasionally missing state changes.

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