Others have run the numbers and compared to PLC scan times, so I won't retread that. Suffice to say, I agree, HSC is realistically the way to go. I do have one thing to add, however, that I haven't seen mentioned yet.
Something to keep in mind as you think about this is that if you want to count the pulses you realistically need both a high signal and a low signal. Thus, if your PLC has a 1ms scan rate (1000hz), the fastest signal you can detect is 500hz.
When I was in school, this was referred to as the Nyquist Theorem. Feel free to google for more info/explanation.
The Nyquist Theorem I did find some info on last night when I googled what I am trying to setup. I still don’t fully understand it, but I’ll do some more reading thanks for the heads up.
The scan time of 1ms is the quickest a normal dc input will get on the ML1400, if I use the HSC’s built into the ML1400 the scan time will become a non issue, from what I understand.