MK3S Firmware 3.10.1 Serial Number issue
So, with the new 3.10.1 firmware, it looks for a valid serial number on startup. If there isn't a valid serial number, it keeps trying until it gets one. So on my MK3S (real serialized machine), somehow my eeprom became corrupted and the serial number shifted about 5 bytes. This means that it could not locate the correct serial number in the eeprom that starts with CZPX, and I could not connect to the printer via the serial port. It would simply timeout. In case anyone else out there comes across this issue, here's how I fixed it.
1. Revert back to the 3.10.0 firmware.
2. From a terminal, such as Octoprint or Pronterface, replace the hex values with your real serial number hex values Hex Tool Here and run this command (this one will print CZPX11111111111111):
D3 Axd15 C20 X435a5058313131313131313131313131313131
3. Re-flash the 3.10.1 firmware.