A MIDI message sent by some MIDI devices all the time to confirm that they are still there and hasn’t gone off line. It carries no other information and hasn’t been widely implemented in the industry. It comes from the early days of MIDI when some companies thought that if MIDI was interrupted it would be a good idea for instruments to change all notes to an off state. When implemented, a MIDI device will usually wait about 300 ms for the active sensing signal to confirm MIDI information is still being sent. If it is not received within this time limit, all notes are shut off and the operation is returned to normal.