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 0x3f8
mknod -m 660 /dev/ttyS1 c 4 65 # base address 0x2f8
mknod -m 660 /dev/ttyS2 c 4 66 # base address 0x3e8
mknod -m 660 /dev/ttyS3 c 4 67 # base address 0x2e8
chown root:tty /dev/ttyS[0-3]

chown(1), mknod(1), tty(4), agetty(8), mingetty(8), setserial(8)