The Z85x30 family synchronous/asynchronous controller chips are used on a large number of cheap network interface cards. The kernel provides a core interface layer that is designed to make it easy to provide WAN services using this chip.
The current driver only support synchronous operation. Merging the asynchronous driver support into this code to allow any Z85x30 device to be used as both a tty interface and as a synchronous controller is a project for Linux post the 2.4 release