For MIDI devices, one statement type that can be used in an INF file is not documented in the Device Information Files section. (For multimedia hardware devices other than MIDI devices, there are no INF file statement types other than those documented in the Device Information Files section; no special switches have to be set for non-MIDI devices.)
If your MIDI device drives a synthesizer that is not on the device, include a statement that sets an External switch in the Drivers key of the registry for your device by using an INF file statement of the form:
HKR,Drivers\midi\msmpu401.drv,External,1,01,00,00,00
Following is an example of such a statement, in the context of the entire section that puts values into the Drivers key for the device (note that this example section is not taken from the sample MIDI.INF file).
[MPU401.AddReg]
HKR,,Driver,,msmpu401.vxd
HKR,Drivers\midi\msmpu401.drv,Driver,,msmpu401.drv
; following is the statement to use for external synthesizer
HKR,Drivers\midi\msmpu401.drv,External,1,01,00,00,00
HKR,Drivers,SubClasses,,"midi"