Need help on Modbus RTU C# Readbyte Error

Question

Hi,

   I have a code which i have been studing but yet to understand one time why will the read byte error occur a lot of time and when i try to run code line by line somehow it wont have problem as the time is not more than time out error. If exceed read byte error will occur due to the duration of time taken, possible to have other potential fix.

 

 

when program run to this program no matter how my stopwatch time keep getting more than the time out value which i stated and by right it shouldnt have.