Thanks for the idea. I tried changing to XON/XOFF, and that didn't seem to help. I went through all 4 different flow control options and none of them appeared to fix the issue. These changes don't seem to appear in port monitoring logs. Is there an other way to determine the flow control being implemented? Or even to determine if that might be my issue?
I also noticed that the XOnLimit and XOffLimit for the stand-alone software seems to be 10 (bytes?), but I cannot seem to set a value that low using the SetBufferSize VI under VISA; they seem to only go as low as 16.
I'm not sure if again I am pursuing a red herring with this or not. I appreciate the advice.
You have the serial settings for the Extech Sound Meter set wrong. Use these settings:
Baude Rate: 2400
Data Bits: 8
Stop Bits: 1.0
Flow Control: None
You do not have to write to the meter to read an output. Instead use the "VISA Read" VI along with the VISA Serial (setup VI) to acquire data. A bit of cropping/formatting of the output should make the number display nicely and in the VISA Serial setup VI you do want a termination character of "A". This A stands for 0xA which is the equivalent to a '\n' character. This method is called "line feed"
One difference I notice in the portmon logs is that the bad log shows a Flush Buffers immediately after the Write (20) while the good log does not flush. Flushing after that write seems like it will eliminate any data - and that matches your results.
That manual seems to omit more than it specifies. Other than baud rate it does not indicate the required settings. The software protocol on the last page certainly does not seem to show any termination character. But it also does not show writing (20) either. What did you do to write that character? Your VI shows *IDN?\r\n which clearly is not what the meter expects.
My bad, my instructions were specified for the Extech Digital Sound Level Meter Model 407750 which appears to behave slightly differently then the model number you are discussing. It is possible to post the serial protocol document for the sound meter you are working with? That protocol document should have some clues on how to adjust the program to properly work