how can i measure frequency use DI?

[kshen]

I have 3 frequency output instrument need to measure, but the DAQ card only has 2 counter. I wonder if i could use DI to measure the frequency? But i don't want to occupy too many computer resource. Do you have any idea about that?  Thanks!

Kevin 

[AnalogKid2DigitalMan]

What range of frequencies? Assume it is a TTL level square wave? Do you have to sample all 3 at the same time

I doubt that you can reliably measure frequency with a digital input, especially if the frequecy is high. You would have to do a fast polling loop (timed) in software, and that could eat up resources, and timing is not all that accurate due to Windows timer.

Do you have analog inputs available, it may be possible to do a sample and derive frequency.

Another option- frequency to voltage converter, but there may some non-linearity errors.

Another idea- multiplex 2 of the 3 signals into one of the counter inputs.

[e-Cash-Money]

Hello kshen,

AnalogKid2DigitalMan gave some really good ideas, especially about the one using the analog input channel.  If you would like to do a hardware timed acquistion of the signal and determine the frequency in software, you can use an analog input channel, or you can use a digital channel if your hardware supports correlated DIO.  The configuration for this operation is a more advanced, and you can find examples and information n how to do it by searching our website.

[AA Dan]

You should probably consider using a PCI/PXI-6601 or 6602.  These have more 4 or 8 counter/timers per board and so you wouldn't need to mess around with other options like frequency to analog conversions (SCC-FV01 or SCXI-1126) which are fine options if you're already using analog signal conditioning.  The 6601/02 are relatively inexpensive and so seem like the best solution for your needs.

Hope that helps out.