HP Princeton HP-21 - Manual (Page 11)

Connect the rear-panel AUX.OUT jack to
amplified speakers, amplifiers or domestic
hi-fi-units for external amplification.
Note: Connect both left(L) and right(R) channels for stereo effect. If you only connect either
one of the channels, the connected channel will only take effect.
Using the Foot Pedals
Connect the plug of the foot pedal unit to the pedal
jack on the rear panel.
Using the MIDI IN/OUT Jack
MIDI stands for Musical Instrument Digital
Interface. MIDI is a world wide standard that
makes it possible for various electronic musical
instruments and other devices.
MIDI IN: Data transmitted from other MIDI
instrument via MIDI is received at this
terminal.
MIDI OUT: Data produced by the Digital
Piano is transmitted to other MIDI instruments
via MIDI from this terminal.
Operation