>> If by blocking function...
In this context, "blocking" means that the current thread is "blocked" or waiting efficiently within the OS. Meaning, the call does not return from the OS until some condition is met. For example, Sleep() is a blocking call which blocks for at least the amount of time given. If your code is spinning in a loop, that's called a "busy wait". This will count against you in the "% CPU Usage" column of task manager.

>> my sequence of cmd and response can take several minutes
Then why not just poll once a second? I see no need for polling the data any faster. And if you are willing to call Sleep(), then just call canReadWait() with a 1 second timeout. Then if data is available it will return immediately. If not, the call will block for up to 1 second.

Is this code multi-threaded? Or are you handling the UI and device interaction all within one thread?

gg