Brother HL-3400CN Series User Manual page 58

USER'S GUIDE
3–4
Notes
When you use the automatic interface selection, note the following:
Once the interface is automatically changed, it is not changed again
for a short period of time. This time period is called "Time Out" and it
can be set with the Mode button. The factory setting is 5 seconds.
The communications parameters [baud rate, code type, parity, stop bit,
Xon/Xoff, DTR (ER), and Robust Xon] must be set for the serial
interface. Although they have been factory set as shown in the
previous table, you may need to change them with the Mode button.
The automatic interface selection function takes a few seconds to
work. If you want to speed up printing, select the required interface
manually via the Mode button.
If you typically use only one interface, we recommend you select that
interface as your default. If only one interface is selected, the printer will
allocate all of the input buffer to that interface.