slattach on N900 to create SLIP interface with NetBSD host

Interestingly N900 seems to have slip kernel module and commands like
slattach to create a SLIP interface.

Is this usable to create a USB network with other end being a SLIP
interface. (In my case the other end is a NetBSD host.)

Which tty device to use as argument to slattach?

If I use ttyS* I get input/output error.

Mayuresh.