How to read serial port only when new data has been sent to it

Question

I've written this very simple vi to read the serial port whenever the MKS Helium Detector sends a new value.  The timing of new data is controlled directly from the front panel of the MKS instrument.  I thought it was as simple as looking at the "bytes at port", and reading the port only if the value is not zero.  The problem I'm having is that the read function is occuring whether I want it to or not.  Consequently, the "Scan from String" function is returning an error (0) once in a while, because it's reading the serial port as it's being updated (I think).   Apparently, when the port is in the process of being updated, it is not zero.   Anyway, I'm assuming there is a simple fix, but I'm having no luck finding the Easter egg.  Any help would be, as always, very much appreciated. 
Attachments:
MKSRead.vi ‏55 KB

Answer 1

You can approach your problem in a couple of ways. The way you've got it written now, as soon as the number of bytes is non-zero, you read and convert. This means that when a partial string is there, you conversion is nor correct. One way is to wait until non-zero bytes like you do now, and then in another loop, read until the byte count is equal to zero and then do the conversion. You can also read until the cr\lf is detected and then do the conversion on the string. You can also enable the termination character for the read but then you'll have to increase the number of bytes to read to something other than just the number of bytes first available. If the instrument always returns the same number of bytes, you can use that as a constant for the VISA Read. Then the read will terminate whenever the byte count OR the termination character is detected.
I modified your VI to show you how you might put it in a loop and what for the CR/LF to be detected. You would want to add a timeout to this so that if the character does not show up, the loop will not run infinitely
Attachments:
MKSRead_mod.vi ‏68 KB