Richard,
You need to learn how the LabVIEW dataflow paradigm works.
Within your while loop anything which does not depend on anything else may run in parallel. So the Stop button and all the other code are parallel. This effectively means that the Stop button will be read at the very beginning of each iteration. Because the mechanical action for the Stop button is set to Latch When Released, it will pop back out as soon as it is read. But the rest of the code, particularly the VISA Read will take about 10 seconds. The while loop will not decide whether to stop or not until everything inside has completed. So, even though the Stop button may have been read seconds ago, the loop will not stop until after the Read. Holding the button down should not make any difference. Once it has been read the value will remain on the wire going to the loop termination terminal until the next iteration.
How can you get around it? You need to make everything else in the loop take less time. But shortening the timeout on the VISA Read may cause problems with getting your data since the device only sends data once every 10 seconds. There are ways to work around that but they can get complicated and the details depend on how the data coming on the serial port is formatted and how you will handle partial messages.
Lynn
