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Why serial ports remain relevant
Even after the introduction of USB, serial ports have not disappeared. The term "serial port" typically refers to hardware that more or less conforms to the RS-232 standard or to corresponding TTL signal levels (TTL signals).
Common use cases
Serial ports are easy to debug during early hardware and software integration. Even after USB became widespread, many peripherals still connect via serial ports. Examples include Bluetooth modules, GPS modules, and various medical devices that use serial links for data transfer.
Options for expanding serial ports
Most embedded systems provide at least two serial ports, but some applications require more. Designers can expand the number of serial ports using several approaches, including USB-to-serial converters, I2C-to-serial bridges, or address/data-bus interfaces to serial ports (for example, using UART controllers).
