That error -1073807346 says given VISA session is invalid. There is a way that could happen.
You have a For Loop wrapped around the VISA write that runs X number of times based on how many elements are in the array coming from the Relay Inputs VI. I don't know exactly what is going on in there. If there are no elements in that array, then the For Loop runs zero times. A For loop running zero times returns the default data for each output tunnel that it has. For a purple VISA wire, that is going to be an empty reference. With the VISA read later on, it will try to read the empty reference and you get that error number.
One way to solve that is to turn the purple tunnels on the For Loop into a shift register. Then it passes through the Valid VISA reference even if the For Loop runs zero times.
I would not recommend a timeout value as low as 100 msec, that is generally not enough time for a serial device to send all the requested data reliably.
I don't think having different loops for the write and read is a good idea. Are you writing a request for data, then trying to read it? That is a typical means of communication on a serial port. If so, you want to have a Read follow a Write so they stay synchronized. No way to do that with two separate loops. And using a global variable to send data from the read buffer in the 2nd loop back to the first is at best unnecessary (nothing else in the 1st loop uses it that couldn't be moved to the second loop), next best just odd and convoluted, worst case leads to race conditions in that variable which will catch up to you later and cause problems.
