FOCUS Enhancements 3.2 - Manual (Page 146)

External Control Commands
132
Callback Commands
mpgCallback
Turns on/off callback (play complete).
Syntax
mpgCallback on
Parameters
on
on=1, send callbacks (default off).
Play Complete Callback
Each time a file ends and another file is started, a string is sent
back to the controller over the serial port. This string has the
following format:
CB 0 channel 2
“0” is the Firefly channel and “2” equals the end of the file.