Visa data read on >0

[Compolitus]

Hi

I have a simple write/ read data to a text file using visa and GPIB “GET” command. The data reads fine but I need to only read in data when the value is >0 to trigger a set number of data points. I’m using visa open, write, read, close and write to text with loops. Unfortunately unable to show VI at this time. 
Any examples would be helpful

[crossrulz]

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@Compolitus wrote:

The data reads fine but I need to only read in data when the value is >0 to trigger a set number of data points.

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And where is this "value" coming from?  We need A LOT more context.

[Compolitus]

Hi crossrulz

Thank you for reply. You are the person that helped me originally. 

The data is from a GPIB frequency counter Visa write sends GET command and visa read is set to input 12 byte ASCII strings. This is then fed to a notepad text document and looped for a total of 256 cycles. 

When run is initiated the cycle starts to read reads all 256 data bytes immediately, even zeros.   I need it to wait until the first proper value byte is received before reading in 256 steps. Greater than zero seems sensible to me as the first value from the frequency generating device is around 9 Ghz. 

I presume that there needs to be some sort of visa “wait until >0” before starting the read cycle. 

[Compolitus]

Hi crossrulz

Are you able to help me with a triggering solution?

Regards

[GerdW]

Hi Compolitus,

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@Compolitus wrote:

I need it to wait until the first proper value byte is received before reading in 256 steps. Greater than zero seems sensible to me as the first value from the frequency generating device is around 9 Ghz. 

I presume that there needs to be some sort of visa “wait until >0” before starting the read cycle. 

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You don't need a "wait"…

You need to implement this pseudocode:

REPEAT
  data := VISARead(device)
UNTIL data > 0
save(data to file)
FOR i=0 to 255
  data := VISARead(device)
  save(data to file)
NEXT

The first loop reads the data UNTIL it is greater than zero. The 2nd loop reads the data and saves to file…

[Compolitus]

Hi GerdW

Thank you. That looks straightforward. Not sure how to insert within my loop VI?  
Can you show example, please. 

Regards

Trevor 

[RavensFan]

You should attach your VI!

[Compolitus]

Hi

Sorry not able to show VI at this time. Will be back in office next Wednesday.

Regards

Trevor 

[Compolitus]

Hi Ravensfan

VI Attached

Thank you in anticipation

regards

Trevor

[cbutcher]

A few questions:

• Why the first loop? If you set N=1 for a For loop, it almost always does nothing. You can just remove it and have the same effect.
  • Here there's a slight behaviour with the wait, but that isn't the most typical way to write it. Use either Stall Data Flow.vim, or a Flat Sequence Structure, or Synchronise Data Flow.vim to add delays in that way
• You're currently reading the string, but presumably you want to convert it to a number. Do you know how to do that? Do you know the format of the string and the type of the output number?

You might consider reading about State Machines - there's a good chance it could be useful for this triggering behaviour. However, it isn't strictly necessary, especially if you only want to trigger once (not repeatedly).

Don't unconditionally write your data if you only sometimes want the values to be stored. Consider also how you want your text file to be formatted (and perhaps if a text file is appropriate, depending on the rate of data acquisition, but I think it should be fine).