Apple //c

This highlights very well, differences between the //c and previous Apple II models:

http://www.applelogic.org/files/IICREFV2.pdf

Built-in "slot" cards

Slots 1,2 - serial ports Slot 3 - 80 column firmware Slot 4 - "mouse interface card" Slot 6 - Disk II

Seems like the serial ports firmware differs, but the hardware is probably the same.

There are a variety of softswitch differences - key ones being: mouse-related (interrupt-driven); VBL ; a few changes in $C01X (functions changed for certain switches); a few other areas.