I have a reputation on this Forum for getting all Hot and Bothered when posters (such as yourself) attach a picture of part of a Block Diagram. This is frustratingly useless -- if you knew where the problem was located, you might be able to show us only the relevant code, but since you have a problem, you should provide us with all of the code, in runnable, modifiable form (i.e. as a VI) so that we can play with it ourselves, maybe alter it, test it, and fix it.
Whew. Glad I got that out of the way.
So I refuse to look at the Shimmer picture. However, the Main VI has some curious bits. I don't know how your Shimmer device works, but most devices that use VISA as a communication medium use termination characters. The reason that they do this is because the receiver of the message doesn't really want to read it until all of the message has been delivered. For example, say the message was 300 characters, and you did a VISA Read at the point the device had sent 30 characters. Great, you ask how many characters are at the port, read 30, get those 30 characters, and they don't make sense (because the remaining 270 characters haven't yet been received nor read).
If you set up VISA to use a Termination Character (you need to see what your sending device uses), then your VISA Read will "block" until it sees the Termination Character (or until the Timeout occurs, which is why there is a Timeout parameter). You then ask how many bytes, see 300, and read them all, knowing you have the entire message.
Another thing I noticed is that you branch the VISA wire inside your For loop and take it to two VISA Write statements (one writes 20(hex), the other writes 00(hex)07(hex). Which writes first? It is indeterminate -- this is LabVIEW, which uses Data Flow, but otherwise is a parallel language, so either could "win the race". Incidentally, I'd advise against using Hex in the String control -- I thought you were writing the string "20" and "0007". If you use the "\" format, instead, the String Constants would be \s (space) and \00\07 (hex 00 and 07). Remember, you want the code to be understandable by any LabVIEW user (including yourself, two months from now, when you've forgotten you used hex here).
Bob Schor
