[CentOS] resetting a serial port

Thu Dec 14 01:45:31 UTC 2023
Robert Moskowitz <rgm at htt-consult.com>

I have Centos 7 arm32 running on a Cubieboard and I am logged in as root 
on its serial uart from another system.

On that system I use

screen /dev/ttyUSB0 115200

Well it was working well for a couple days, but now only garbage comes 
across.  Something messed up the serial link.

pulling the usb cable to the usb/serial adapter does not reset things.

I can ssh into the server and see root logged into ttyS0

How do I reset that serial port so that I can work on the system?

thanks