I wanted to have a cartridge port to make it easier to load new programs into ROM.  It also makes it more like old Commodore 64 or Atari 400/800 computers.  I don't think there will be many cartridges made, but I like to think that it's possible.  I also want to be able to pop out the cart and reprogram it easily without modifying the computer itself.  When the cart is inserted, the /ROMEN line is forced high by the cart which disables the internal ROM.  I also included the data bus, clocks, and an /EXPAN input which is enabled with /IE just in case someone comes up with a good idea.

I chose an edge connector that is keyed so that it's not possible to plug the cart in backwards.