Frequency meter in PLC

Hello, Im trying to measure the frequency (Hz) of a fast input.
what Im doing now is copying the Acc Count twice a second apart and
subtracting. This seems to work but it takes 2 seconds to update the frequency value which may or maynot be ok. Is there a better way of doing this? thanks.

I forgot to mention, Im working with a DL06 PLC

Why not try a converter. Companies like Endress and Hauser do digital to frequency signal converters where you get a 0-10V or 4-20mA signal to feed into your PLC.


So how fast a frequency are you talking about.

240Hz Max

Favian,

I found this in the PLCS.net download section
http://www.plcs.net/downloads/index.php?PHPSESSID=bfd9084d96e7d450f6fb743b5d7ef61d&direction=0&order=&directory=Automation_Direct&PHPSESSID=bfd9084d96e7d450f6fb743b5d7ef61d

look for RPM 05 zip.

[font=Arial, Verdana, Helvetica, sans-serif]I have no Idea if it works. I don't have a plc on my desk to try it out. [/font]

I assume you are trying to calculate a value that updates continuously. Create two registers, current_count and prev_count. Once every second, copy current_count to prev_count, move the accumulated value of the counter into current_count, and subtract prev_count from current_count to get the current freq.

If you do it this way, you'll be updating every second instead of every 2 seconds. If that still isn't fast enough, how about updating every 500ms and multiplying the result by 2?