Doing VISA Reads to an Instrument depends critically on understanding and following the Manufacturer's "Instrument Guide" that describes the Communication Protocol and Command Structure (I've wrestled with this problem with a number of different instruments).
What has always worked well for me is to start by not using LabVIEW, but by using MAX (the Measurement and Automation Explorer). MAX can "talk" to a VISA port, which means you can test all of the following:
- Setting up the VISA port with Baud Rate, Parity, and other Serial parameters.
- Set Flow Control as per Manufacturer (when in doubt, use None)
- Set I/O Settings, especially Termination Character if Device uses it. You'd like Send End on Writes, Enable Termination Character, use it on Reads, and make it Line Feed (but check your Manual to see what your Instrument sends).
- Once you've "set up", go to the Input/Output section and start trying to Send/Receive Commands. You'll see it is set up to send "*IDN?\n" (which I think is a query, Please tell me your ID, terminated by a Line Feed). If you click "Write", this gets sent to your Instrument. Then click Read, and see if you get anything back. If not, you'll get a Timeout or some other such error. Now the fun begins -- figuring out what went wrong ...
Using MAX, you can configure your VISA Port, try to "talk to" and "listen to" your device, and if there are errors or unexpected results (like you send "Hello", and it says "iasj*32:#S", i.e. gibberish"), you know something is "not quite right" (gibberish is often "Wrong Serial Parameters" -- pay attention to Baud Rate (easy to get right, just by following the manual) and Parity (this one can fool you ...).
Bob Schor
