Here is an old (1995) favorite serial port hardware reference
http{delete this space}://{delete this space}www.{delete this
space}sytekcom.com/eng/SerialPort-hardware.{delete this space}html.
Most all of the UART register set, and more, is accessible using POSIX
termios functions.
I have looked hard at Arduino, Raspberry Pi, and other popular
microcontroller boards. Nothing I found compares to the plus and minus
12 volt RS232 line drivers. Nothing I found compares to the interrupt
handling of many, many UART events.
Also interesting is how Linux can now use new shared high interrupts
with PCI UARTs.
I don't know how industry will grow without automation. And I don't know
how automation can grow without Linux and the serial port.
I hope somebody copies and saves the above web document.