Each of the EDUs, {HEAD ~ keypad controllers} and CCIs of {TAIL ~ the system} is connected to a common communication bus 
Each of the EDUs , { HEAD ~ keypad controllers } and CCIs of { TAIL ~ the system } is connected to a common communication bus
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH