$ modprobe slip
  $ slattach -L -p slip -s 57600 /dev/ttyS0 &

This should create an interface with a name like sl0, or sl1, etc. Add -d to get debug output. This will show the interface name.

NOTE: The -L option is included to suppress use of hardware flow control. This is necessary only if you are not supporting hardware flow control on the target.

NOTE: The Linux slip module hard-codes its MTU size to 296. So you might as well set CONFIG_NET_ETH_MTU to 296 as well.

  $ ifconfig sl0 10.0.0.1 pointopoint 10.0.0.2 up
  $ route add 10.0.0.2 dev sl0
  $ tcpdump -n -nn -i sl0 -x -X -s 1500

NOTE: If hardware handshake is not available, then you might try the slattach option-L which is supposed to enable "3-wire operation."