You write your command to the port, then immediately check the bytes at port and read those. Now since the time delay between these two operations are 0 no bytes have yet arrived at the port and so the read function is asked to read 0 bytes...
If you know how many bytes the reply will consist of do not check the bytes at port but wire that number of bytes to the read, it will then wait up to 20 seconds (if I recall the VISA behaviour correct) for that number of bytes to be available. OR continously check the number of bytes and do a read when it reports the correct number of bytes (then you can make your own timeout...). If the reply has a termination character VISA can be set up to return when it has received that.If you do not know the length of the reply nor have a termination character to rely on you can define a rpely timeout and an interbyte time limit, check the number of bytes every interbyte time and if the bytes starts to come in but there is a halt longer than the defined limit you assume the whole reply has arrived and do the read...
