XwirePhys nn [D>=28]

On controllers with programmatically selectable serial interface, select which electrical interface to use. The argument nn makes the selection:

n = 0 gives the default interface (typically TTL)

n =1 gives the alternative interface (typically RS485)

Don't confuse this with port selection.

Xwire documentation

Dialect restriction: This instruction is not implemented on boards prior to dialect D=28.