I'm getting occasional bad counts on my pcie 6323 that cause the instrument to drift out of position with where the NI board thinks it is. Although the signals out of the encoder do not look that noisy, experimenting with digital PFI line filtering has not made much difference.
I suspect the problem is that I've tied the negative differential line of both phases to the common digital ground on the NI board, negating the noise immunity provided by differential signaling. Can anyone recommend a 5v 4MHz differential line to TTL adapter suitable for use with a NI PFI input? I see some discussion in old threads, but the product links are discontinued or dead.