Since you don't show rest of the code, only guessing I can make.

1. Try to short tx-rx of the USB converter to make sure it is working
2. The 22K maybe too large. Try a 4K7
3. The array textline needs to be filled with 8 characters before serin exits. If you type less then it just waits there forever
4. Serin will hang if the voltage input is not correct. Even if you add timeout....!
5. Are you sure your display subroutine is OK?

Ioannis