Just as an FYI for anyone who’s having trouble upgrading your MakerBot Cupcake CNC plastruder firmware, the instructions built into ReplicatorG don’t tell you to unplug the TTL cable from the motherboard and plug it into the extruder board.
So, save yourself 5 minutes of frustration and googling for the answer… and just plug the TTL cable into the extruder board. :)