How do I use a Counter input to acquire frequency data that is slower than my sample rate?

I am trying to write an application with the Data Acquisition Toolbox that acquires a shaft speed via a counter input. The shaft speed sensor input to the counter channel will vary from 0 (stopped) to 1600 Hz. I will also be acquiring several standard analog inputs simultaneously.

I want to be able to acquire the data at a fixed sample rate of 2000 Hz. Any time my counter input signal frequency is less than my sample rate, I get a hardware error:

"Multiple Sample Clock pulses were detected within one period of the input signal. Use a Sample Clock rate that is slower than the input signal."

This is of course kicked because there are less than 2 and sometimes no edges counted in the interval of each sample.

It appears that I need to be able to dynamically vary my sample rate as a function of the counter input frequency such that my sample rate is always slightly less than the counter input. This doesn't work for my application since I need to acquire the other analog signals at a higher sample rate.

I could set up a secondary session that only acquires the counter input data at a slower frequency, but then I wouldn't accurately measure transients in the counter frequency that occur quickly.

I also tried using another counter output to generate the sample clock based on the current value of the shaft speed counter input, but I found that counter inputs cannot use an external sample clock.

DAQ hardware is a NI PCIe-6321.

Simple representation (in practice I'm doing this in a GUI via background acquisition)

    s = daq.createSession('ni');
    s.addAnalogInputChannel('Dev1','ai0','Voltage');
    s.addCounterInputChannel('Dev1','ctr0','Frequency');
    s.Rate = 2000;              
    s.DurationInSeconds = 10;    
    [data,time]=s.startForeground();

Any creative solutions would be appreciated.

Answer 1

[Hans Vertongen]

I got the same problem. Did you find an answer yet?

Answer 2

[Stephan Stranick]

You need to have your input connected to the right terminal. You need to use counter 0's gate input, not the source input.