Just wanted to share a solution to a problem that had me banging my head for almost a week.
I got my Pi and Uno all set up. Many thanks to LBussy and RandR+ for their hard work. I got one of my flow meters tested and ready to go. I take the rig down to my keezer and hook it up to my monitor, keyboard, mouse and keg full of water for testing. The Uno is running on its own dedicated power supply. Suddenly pours aren't being registered. I look at the Uno IDE and there's a few letters over the serial monitor, but not the normal "alive" message. Look at the log and see errors that service is ready, but its not seeing the Uno.
So I start troubleshooting this. I replace the USB cable to the Uno. No luck. I'd unplug and re-plug the Uno, and it would work great, but on a different serial port (ACM1 rather than ACM0). I do see pours being registered in the Uno IDE, but not in raspberry pints. Reflash the Uno, still having the same problem. Unplug everything and take the pi and Uno up to my home office to try and troubleshoot more. I SSH into the pi intending to pull the rpints log. I look at the log, and no errors! I take it back down to the keezer, but I don't hook a keyboard and mouse into the pi. I re-connect my ssh session and look at the log. No errors again. I hook up my test flowmeter, pull a test pour, and it works!
TLDR: Flow meter connection from Uno didn't work while keyboard and mouse were plugged into the pi. No keyboard or Mouse, flow meter works. Keyboard and mouse, flow meter doesn't register.
I know pi's are sensitive regarding power supplies, but I didn't ever thing that a keyboard and mouse would draw enough from power from the USB ports to mess up the Uno communication. The pi was a hand me down from a co-worker who got one of the new pi 4's, so I just took it as a given that the power supply would be strong enough. Oh well, I can reach the admin panel from a different pc and dont' need to keep a keyboard and/or a mouse plugged in all the time.