For transformer-isolated output of mixing bus signal to feed other
mixers, amps and tape recorders. (Functions only in A-900MK2, M-900MK2, W-900A Series,
700 Series and BG-M Series; not usable in P900MK2 Series)
Generates one of eight tones via switch-closure. Can be used for
doorbell, paging, door/gate release, telephone night ringer, annunciator,
class change or pre-announcement tone.