Roland UM-3EX Owner's Manual page 41

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ON/OFF Settings of the THRU Switches
You can use the THRU switches to select the transmission destinations for MIDI messages
from the devices (MIDI keyboard, digital piano, etc.) connected to the MIDI IN connectors.
There is no need to change any cable connections.
Switch Setting
OFF
ON
fig.thru-e.eps_60
* Don't change the setting of a THRU switch while playback is in progress. Doing so will cause MIDI
errors.
MIDI Message Destination Device
MIDI messages are transmitted to your computer (sequencer software).
MIDI messages are transmitted to the MIDI devices (e.g., MIDI sound
module) connected to the MIDI OUT/THRU connectors.
MIDI Message Flow (THRU OFF)
MIDI Message Flow (THRU ON)
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