Wel If the Question is HOW IS THE DATA TRANSFERED BETWEEN THE DEVICES Then the answer is

Code:
Keyboard ps2/AT/USB = Serial
Mouse ps2/serial/USB = Serial 

Printer            25pin    = Parallel
                        USB     = Serial

HDD                 *IDE    = Parallel
                        SCSI    = Parallel
                        SATA    = Serial  (this is the new fast breed)
Now I hope this has helped you with your homework

Cheers