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today I downloaded the update for 0.5.21 version on my raspberry PI3 and discovered that serial /dev/ttyS0 does not exist anymore.
What's the new name of it and why has it been moved?
It was never used in emteria.OS. If you need it, you can enable it in the config.txt (https://www.raspberrypi.org/documentation/configuration/uart.md).