System Exclusive - the only SysEx messages currently recognized are the SP-MIDI MIP message, and the General MIDI On/Off message.
The General MIDI On Command is:
0xFF, 0x7E, 0x7F, 0x09, 0x01, 0xF7
The General MIDI Off Command is:
0xFF, 0x7E, 0x7F, 0x09, 0x02, 0xF7
Please see the MIDI Specification for more information on these commands.