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USB Interface Mode vs. Merge Mode vs. Thru Mode
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The MIDISPORT 2x2 Anniversary Edition has a MIDI Thru-USB/Merge button which selects whether the device is in MIDI Thru Mode (button set to the ‘out’ position), in Merge Mode (standalone operation; button ‘in’) or in MIDI Interface Mode (used with a computer; button ‘in’).

USB Interface Mode
MIDISPORT 2x2 is in USB Interface mode when the MIDI Thru - USB/Merge button is set to the ‘in’ position and the device is connected to and initialized by an active host computer via USB.  In this mode the input and output ports appear as discrete inputs and outputs in your music software and are completely independent of each other.

Merge Mode
In Merge Mode, MIDISPORT functions as a standalone device (i.e. it does not operate as an interface to a computer).  All MIDI data from the In A and In B ports is combined (merged), and this merged stream is output identically on both the Out A and Out B port.  In this scenario, power must be provided by an optional USB-type external power supply, and the MIDI Thru - USB/Merge button must be set to the ‘in’ position.

NOTE: In some cases, you can use Merge Mode while connected to a computer via USB, but only if the operating system is not currently running.  In this scenario, the computer serves only as a power supply while MIDISPORT is operating as an independent device.  This is possible, because some computers provide power to their USB jacks even when shut down or in sleep mode.  Please note that it may be necessary to disconnect and reconnect the USB cable after shutting down the computer for MIDISPORT to disengage USB Interface Mode and enter Merge Mode.  Check the documentation associated with your computer to see if it provides power to its USB jacks while shut down or in sleep mode.  A separate USB-type external power supply is not required if MIDISPORT is connected to such a jack.

MIDI Thru Mode
MIDISPORT 2x2 is in MIDI Thru mode when the MIDI Thru - USB/Merge button is set to the ‘out’ position. In this mode, MIDISPORT passes the In A data stream directly to Out A and the In B data stream directly to Out B.  This mode bypasses connected computers and does not require power to operate. Thus, MIDI Thru mode can always be enabled, regardless of whether or not MIDISPORT is connected to a computer, or a power source.