How do I pragmmatic​ally determine what comm ports my serial instrument​s are on?

Question

I am building a test stand that incorporates 5 serial instruments; 2 digital loads and 3 power supplies. I can use my PC's device manager to figure out which comm port each device is on and set the VISA Resource Name so everything runs as expected. All is well at this point. The problem comes in when an instrument's USB connection is unplugged and replugged, at which point the system may renumber the port assignments; the result is that my program errors out, I have to figure out what port everything is on all over again, and change the VISA Resource Name for all of my instruments. While this is a minor annoyance for me as the developer, this would make the program unusable by the user.
While "don't ever unplug an instrument" is a sensible answer, the fact is that these things happen from time to time - like for calibration - so I want the program to work regardless of instruments being unplugged.
Is there a way to have the vi automatically identify what port each instrument is on?

Answer 1

I've used a USB connected barcode scanner that appears as a COM port. If it is left connected to the same USB port the COM port remains constant but if it is disconnected and then put back into a different port the COM port changes. This sounds similar to your problem so I'll share how I solved things for my application.
The scanner I use accepts commands and will respond with a known string when I send a character 'I' via the serial connection. Using this I can see if the scanner is connected to the expected COM port (default port is stored in an ini file).
If this fails I create an array of possible serial ports by trying to open COM1 and checking for an error. If this does not generate an error I add it to an array. Next I try the same with COM2, COM3 up to COM16 (for example). This gives me a list of COM ports I can open.
I then iterate through the array trying for a response and stopping either when I get the expected response and continuing with the application or if the device is not found by the time I get to the end of the array I show the appropriate error condition to the operator.
If your instruments can return some kind of ID that will allow you to identify them as mentioned in another reply then this method could be used. Your 3 power supplies and 2 digital loads would need to be able to return a unique identifier otherwise you wouldn't be able to tell them apart and I think you'd possibly run in to problems.