The background rack is a bit unique, since racks in songs can’t use background rack ports, so routing is a bit fiddly - but it can be done. The trick is to use the “loopback” feature of Cantabile, which allows you to allow MIDI input ports as output ports to allow Cantabile to feed MIDI data back into its input ports.
So I use the background rack to filter and translate data from my physical devices (e.g. Kurzweil PC3K, AKAI MPK249, my custom “Magic Keys”) to standardized abstract “upper keys”, “lower keys”, “pedals”, “drum pads” and some internal magic ports to feed my songs. These abstract MIDI input ports aren’t connected to any physical MIDI interface - they only get fed by the background rack.
So the background rack translates the input from my device input ports and feeds it into the “Loopback - Main Keys”, “Loopback - Second Keys”, “Loopback - Drumpads” etc. My input racks then receive data from these ports (“Main Keys”, “Second Keys” etc.). I could also use these abstract input ports directly in my songs, but I’ve gotten used to using my abstract input racks, so I’ll continue to use them for better visibility, even though most of the translation work is done by the background rack these days.
If you want to dive deep into this rabbit hole, here is a post that gives more detail:
Cheers,
Torsten