the absence of a transition in the data signal thus for example a string of 1 39 s would result in a long period without signal transitions in order to force transitions in the data signal a bit stuffing technique is used to insert a zero after a sequence of consecutive 1 39 s of a prescribed length e g after a sequence of six consecutive 1 39 s accordingly if a device receives a sequence of consecutive 1 39 s that exceeds the prescribed length the device may conclude that an error has occurred and ignore the data packet by way of example the data transfer technique of fig3 may be used to reprogram a processor embedded in system controller 18 therapy control module 14 patient parameters module 22 or user interface module 24 program data such as a software upgrade may be transferred via system bus 20 to the device to be reprogrammed the software upgrade may then be stored using for example a ram device or a flash memory the data transfer technique of fig3 can also be used to control the functions of the various modules of defibrillator 12 for example system bus 20 can be used to effect the delivery of therapeutic shocks to patient 16 via defibrillator electrodes in this mode of operation operator 10 uses the external keys to activate.