NI-9401 PWM with FPGA

[Lysandros]

Hello,

I use the NI-9145 EtherCAT Expansion Chassis together with a NI-9401 Digital I/O module to generate a pulse train with a period of 2ms and a duty cycle of 1/4. So I implement a Loop timer of 80000 ticks (2ms with the 40MHz clock) and a Wait of 20000 ticks (0,25ms with the 40MHz clock).

The FPGA.vi works, but in the oscilloscope the pulses appear very rounded off, as if they were passed through a low pass filter. It looks as if the rise time is considerably higher than the 100ns written in the the datasheet of the NI-9401.

Does anybody have a suggestion / idea for this issue?

Best regards and a nice weekend

Lysandros

[nathand · Accepted solution]

What did you connect to those pins? Most often when this happens, you are asking the outputs to provide more power than they're designed to provide. Try putting an oscilloscope on it with nothing else connected to the outputs.

[Lysandros]

I connected the DIO-0 & COM pin of the NI-9401 at a Hameg-205-3 oscilloscope, which is rated at 20MHz and has a high input impedance.

Later I connected it to an analog NI-9215 and was only seeing +5 and 0 volts in Labview. But since the scan engine runs at 1ms, I am not sure if the time resolution is not high enough to capture the intermediate signal levels, or if the rise time really is fast and my oscilloscope settings are actually the problem.

I will check on Monday all the settings again however.

Thanks

Lysandros89