Timebase for 6251 counter

[crdust]

Hi we are doing an edge count of a 10Mhz signal. I'm using the on demand mode of edge count. What is the timebase(Frequency  of counter clock)  the counter and how is it selected.

-Charles

[Kevin_Price]

In LabVIEW, there are DAQmx property nodes that will let you set or query the timebase.  Probably available in a Channel nodes if you dig down a ways in the menus.

I've gotta ask though, ahat do you intend to do with this timebase once you learn how to select it?   Because the notion of a timebase doesn't really make sense to me for an edge-counting task.  You're simply incrementing a count as edges come in.  The timebase doesn't get involved. 

-Kevin P.

[crdust]

Hi we are using Labwindows. The purpose to know the timebase is to ensure that it's a higher frequency than

that of the signal whose are being counted. In our case the signal frequency is 10Mhz. We are using the M series 6251

DAQ card. So if the timebase of the counter is 20Mhz or 80Mhz we should be ok.

-Charles

[Kevin_Price]

It sounds like you're thinking "Nyquist", which doesn't really apply to TTL edge-counting operations.  The board isn't trying to digitally sample the incoming 10 MHz signal.  It simply has circuitry that increments a count register on every incoming TTL edge. 

The speed spec that matters is named something like "maximum external source frequency".   Hmmm, I couldn't find any such spec in the M-series detailed specs or online manual.  Best guess is that it's around 80 MHz to match the max available internal freq.   I surmise this based on the datasheet for the similarly functioning counters of the 6602.

-Kevin P.

[Mark_E]

Hi Charles,

The PCI-6251’s specifications manual is a great resource. On page 9 it indicates that the internal base clocks are 80MHz, 20MHz, and 100kHz. The last two are just divide-downs of the master 80MHz clock. Post again if you have further questions.

[crdust]

I'm just hoping if you could confirm that the default timebase for the counter of a 6251 device

is 80Mhz. The documentation is ambiguous in this regard.

-Charles

[Mark_E]

Yes, the maximum timebase is 80MHz for an M-series card. It is selected automatically based on some different parameters such as the input frequency. You can read what value it is currently set to with a property node. A list of the property nodes available are found in the NILabWindows/CVI Help (Start -> All Programs -> National Instruments -> LabWindows CVI X.X -> LabWindows CVI Help) under “list of timing properties”. Look under Sample Clock -> Timebase.

There are also a few different counter methods. A good resource for learning about what they are and when to use them is the NI-DAQmx Help (Start -> All Programs -> National Instruments -> NI-DAQ -> NI-DAQmx Help). Look under NI-DAQmx Key Concepts -> Counters.

If you have other questions, let me know.