ttys - serial terminal lines
ttyS[0-3] are character devices for the serial terminal lines.
They are typically created by:
mknod -m 660 /dev/ttyS0 c 4 64 # base address 0x03f8
mknod -m 660 /dev/ttyS1 c 4 65 # base address 0x02f8
mknod -m 660 /dev/ttyS2 c 4 66 # base address 0x03e8
mknod -m 660 /dev/ttyS3 c 4 67 # base address 0x02e8
chown root:tty /dev/ttyS[0-3]
/dev/ttyS[0-3]
mknod(1), chown(1), getty(1), tty(4)