The upstream and downstream ports are defined by the presence of Pull-Up and
Pull-Down reistors at the buffer. These components are internal to the
ADuM4160 and can not be disabled during normal operation. Reversing the upstrean
and downstream ports would leave the host side with no pullup, and
the peripheral side with double pull up resistors. This would probably make
enumeration impossible. The upstream side must always be connected to a host
and the downstream side must always be connected to a peripheral.
The ADuM4160 can act as a single speed host interface, but the resistors will
not let the part be turned around.