Trouble measuring frequency using a FP-CTR-502

[jwmcclure]

I am trying to measure 8 frquencies using a FP-CTR-502. Since I need 8 channels I cannot use the hardware timer method of generating a 1kHz reference pusle like the examples use because that would consume one of my channels. What I tried to do was read the counts from the 502 along with the timestamp. Then I reset the counters and recorded the timestamp then. From there I subtracted the timestamps (previous reset from current read) and divided that into the count to obtain frequency. I set it up in a sequence structure to make sure the counters were reset after they were read. The whole thing is in a loop repeating at 1 or 2s. I seem to be getting eratic results on a slow computer. Is this method valid? Is the time stamp fr
om the FP hardware, or is it fromt he computer clock? Could the computer somehow cause this problem? Any help would be appreciated.

[Mike S. @NI]

The origin of the time stamp depends on the type of network module you are using. The following discussion covers how time stamps are generated with the FP-1600 and FP-1000:

http://exchange.ni.com/servlet/ProcessRequest?RHIVEID=101&RNAME=ViewQuestion&HOID=50650000000800000020230000&ECategory=Distributed+I%2FO.FieldPoint

My best recommendation is to purchase another CTR module. Using software timing will usually give you erratic results. That being said, make sure to use the "tick count (ms)" function if you must use software timing. That is the most accurate method for software timing. Also, you should set the counters to reset on read as recommended in Method 2 of this KnowledgeBase:

http://ae.natinst.com/operations/ae/public.nsf/fca7838c4500dc108
62567a100753500/d8db42f9fdc3607e862568640047f144?OpenDocument

Regards,
Michael Shasteen
Applications Engineering
National Instruments
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