Maximum Count Value

[Shahrukh]

I have a NI-6220 with a 32-bit counter.  For my application, I only want a counter to count from 0 to 999999 and then rollover.  Is there a way to set this either in MAX, or any DAQmx property nodes, or do I just have to make my own logic within Labview. 

[Kevin_Price]

You can't make the hardware do exactly that, but there are some other options that may be acceptable:

1.  You can use the "quotient & remainder" math function and only display the remainder when you divide the actual count by 1 million.  However, if the actual 32-bit count might also roll over, you'd get a discontinuity when it happens.

2.  A hardware-based method may work depending on characteristics of the signal you're counting.  It's a little bit involved if you're new to counters.  A basic outline is that the counter is configured as a two pulse encoder.  You give it an initial count of 999999.  You enable z-index reload with a z-index reload value of 999999.  You configure a counter property that causes it to "pulse on terminal count."  You wire that output signal back in as the z-index pulse.  You wire your external signal as the B input so the counter will count down from 999999 to 0.  Finally, your software will return the value 999999-actual count in order to look like it counts up from 0 to 999999.

-Kevin P.

[Shahrukh]

I actually thought of the Quotient/Remainder solution right after posting that question.  It works quite well as I count up, but as you mentioned, I have a discontinuity if I count down and it rolls over.

I'll try to do some of the hardware tricks you mentioned to see if that fixes the problem.  For now I am just starting the counter at 1000000, which shows up as 0 for me and then I can count down atleast that many pulses before the actual roll over.

Thanks for the help. 

[Kevin_Price]

Hold on!  I just remembered an easier way!

Take the unorthodox approach that instead of setting up a counter input task, create a continuous pulsetrain output task with units = Ticks.  Configure your external signal as the "Tick Source".  Specify that initial delay = low time = high time = 1 million.

The internal behavior of a counter in pulsetrain generation mode is to count down from # low ticks to 0.  When it reaches terminal count, the output state toggles and the count reverts to # high ticks.  It counts down to 0, the output state toggles, and the count reverts to # low ticks.   Etc.

I am not 100% sure a DAQmx Read vi will be allowed, but you definitely can query the count value using one of the DAQmx property nodes.  Probably a channel property.  Then you'd just do the previously-described software trick of reporting 999999-actual count so the reported value counts up from 0 to 999999.

To make a continuous pulsetrain, put a DAQmx Timing vi in the config chain and specify "continuous sampling" -- or just adapt a shipping example.

-Kevin P

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