You don't need to reinitialized the serial port every iteration of the while loop.
You don't need to set the End Mode for Writes before every VISA Write. You probably don't ever need to set it at all.
Move those things outside the while loop.
Beyond that, I don't know why you'd need use highlight execution, unless your equipment can't respond that quickly with commands, or the 50 msec wait between writes and reads isn't fast enough for the to instrument to create a response thus yielding zero bytes at port then nothing to read in the VISA Read.
Fix the earlier things I said. Then try to have a longer wait between the Write and the Read.
Does your device send a termination character? You disabled it with your Serial Initialization. But the format of your Write commands putting one on the end usually means the device will responde with one. If the device does send a termination character, then enable it. Get rid of Bytes at Port and read a sufficiently large number of bytes on the VISA Read.
Why do you do the first VISA write in the loop if you never do anything with the data that you get back from its VISA Read?
