Epson DLQ-3500 Reference Manual page 72

I/F (interface) mode
The printer provides a slot for an optional interface in addition to
its built-in parallel interface and USB interface. You can choose
Auto (automatic), Parallel, USB or Option for the interface mode.
When you select Auto, the printer automatically selects the
interface that is receiving data, and uses that interface until the
end of the print job. The Auto setting allows you to share the
printer among multiple computers without changing the
interface mode.
If only one computer is connected to the printer, you can select
Parallel, USB or Option, depending on which interface your
computer uses.
Auto I/F (interface) wait time
When the printer is in the auto interface mode and it receives no
data from the currently selected interface for the amount of time
specified with this setting, it determines which interface is
receiving data and switches to that interface. You can specify
either 10 seconds or 30 seconds for the auto interface wait time.
Parallel I/F bidirectional mode
The Parallel Interface bidirectional transfer mode is used when
this feature is on. If bidirectional transfer mode is unnecessary,
turn this off.
Packet mode
Be sure to select Auto when printing from Windows applications
using the printer driver included in the CD-ROM that comes with
the printer. In case you encounter any communication troubles
when printing from applications based on other operating
systems such as DOS, turn off this mode.
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Control Panel