I thought it would look like a typical HID, like the old mice/keyboards used serial IO.
When a Wacom serial tablet is connected to a serial port, software can interact with it directly by opening the appropriate device, usually /dev/ttyS0. The Wacom X driver does precisely this, and all stylus movements are converted into X input events for programs like the Gimp to turn into fluid brush strokes.
Wacom tablets are capable of handling a number of different data protocols, and the Linux Wacom Project code currently utilitizes two,
Wacom Protocol IV and Protocol V (our UD-08-08 seems using Protocol IV ... not sure). Each operates with a fixed-sized packet, the length of which depends on the model and ROM version.
When the serial tablet is reset, it defaults to a standard baud rate, often 38400 baud (our UD-08-08 works at 9600). From there, the device type can be queried, and if the tablet supports it, the baud rate increased to a higher value. Additionally, model parameters like tablet size can be queried directly to determine which features are available.
Once configured, the tablet streams data back to the application as tools are brought into and out of proximity, are pressed against the tablet surface, or are tilted or inverted.