See the Kerberos page for details.
All cartridges are sold out and I don't plan to produce any more.
Currently I'm working on a new cartridge, which will have MIDI as an optional feature, see here. You can follow me on Google Plus for announcements of it (and you'll get a few unrelated geeky posts), or subscribe the Kerberos MIDI Google Groups, where I'll announce it, too.
26. January 2016, Frank Buß