Could writing that command without sending a number be a way to query the current value in the instrument without changing it? Because it looks like you want to read the current value, compare it to the dial setting, and only write out a new value if it is different.
However, your VISA read makes no sense. You have a blue wire connecting the VISA write to the VISA read. The output of VISA write is # of bytes written. You feed that value into the VISA read input which means # of bytes to read. I really doubt that you want to read exactly the number of bytes you have written out. If you try to do that, you are probably either getting a timeout because the # of bytes returned are less than what you are asking for. Or getting an incomplete message because there are more bytes being returned than what you are asking for.
As Smercurio said, take a look at the syntax for what you are trying to do with that first command, and look up in the manual for what the response should be that you are looking for in the VISA read as I don't think you have that wired up properly.
