Unable to count fast pulses with DAQ(pcmcia card)AI-16E-4, using the example vi

[nelsons]

I am using the AI-16E-4 PCMCIA Daq card to count pulses from a device that has an open collector pulse output. I am current just trying it out to see if I can get it to count the pulses correctly before I go ahead and further develop the program. I am using the Example program for counting pulses with this device.
What I am finding is that when I increase the pulse rate above about 20,000hz it slows down and then stops counting pulses. The documentation for this device states the max input frequency is 20mhz, therefor it should be able to count the pulses I am planing on providing. The fastest rate the device I am connected is around 2.mhz. Can someone tell me how to get the AI-16E-4
to count these open collector pulses.

[Filipe A.]

Hello;

You need, first of all, to provide a pull up resistor for the open collector device, in a way that its high state fits within the TTL levels.
Once that is done, you just connect its output to the counter source input. The device should be able to go up to 20MHz, with no problem.

Regards
Filipe A.
Applications Engineer
National Instruments