Yamaha FX900 - Manual (Page 7)

External Control
For further versatility and smooth integration with MIDI-based
music systems, the FX900 offers broad range of external
control capabilities. Any of its 100 memory locations may be
directly selected from a remote MIDI device such as a
keyboard or MIDI foot controller. It is also possible to directly
control up to four different effect parameters simultaneously
in real time, either via MIDI controllers, foot controllers
connected to two rear-panel jacks, or a combination of MIDI
and foot controllers. You could, for example, control the
modulation depth of a flanger effect and the reverb time of a
reverb effect in real time as you play! See page 45 for
operational details.
Comprehensive remote control is possible with the Yamaha
FC900 Foot Controller — designed specifically for use with
the FX900. The FC900 provides effect program selection,
effect group switching, bypass switching, parameter control,
and other versatile capabilities in a sturdy floor unit. Consult
the FC900 literature for further details.