Setting up digital piano with external sound module.
You can use the EG2280USB/EG8280USB to control external sound modules via MIDI.  This allows you to use the keyboard and pedals of the digital piano to control the sound generators of other synthesizers or sound modules not equipped with a keyboard.  When you also connect the audio signal of the external sound module to the EG2280USB/EG8280USB.  All sounds played in addition to the sounds of the EG2280USB/EG8280USB can be heard through the internal loudspeakers.  
1.  Connect the MIDI OUT jack of the EG2280USB/EG8280USB to the MIDI IN jack of the external sound module.
2.  Be sure to set the MIDI transmit/receive channels of both instruments to the same number.
3.  Connect the audio outputs of the sound module to the LINE IN jacks of the EG2280USB/EG8280USB, if you want to play back through the digital piano loudspeakers.