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Chapter 8: GPS and a Real- T ime Situational Displa y           209



                              Pin Number   Name(s)             Description
                              1            Vcc                 Power supply, 3.3 V
                              2            Dout                Data out (TXD)
                              3            Din                 Data in (RXD)
                              4            DIO12               GPIO pin 12
                              5            Reset               XBee module reset, pin low
                              6            PWM0/RSSI/DIO10     Pulse width modulation (PWM) Analog 0, received
                                                               signal strength indicator (RSSI), GPIO pin 10
                              7            DIO7                GPIO pin 7
                              8            Reserved            Do Not Connect (DNC)
                              9            DTR/SLEEP_RQ/DIO8   Data Terminal Ready (DTR), GPIO Sleep Assertion
                                                               (pin low), GPIO pin 8
                              10           GND                 Ground or common
                              11           DIO4                GPIO pin 4
                              12           CTS/DIO7            Clear To Send (CTS), GPIO pin 7
                              13           ON/SLEEP            Pin high when NOT sleeping
                              14           Vref                Voltage reference level (used with analog-to-digital
                                                               conversion)
                              15           ASSOC/DIO5          Pulse signal when connected to a network, GPIO
                                                               pin 5
                              16           RTS/DIO6            Request To Send (RTS), GPIO pin 6
                              17           AD3/DIO3            Analog Input 3, GPIO pin 3
                              18           AD2/DIO2            Analog Input 2, GPIO pin 2
                              19           AD1/DIO1            Analog Input 1, GPIO pin 1
                              20           AD0/DIO0/COMMIS     Analog Input 0, GPIO pin 0, Commissioning Button


                             Table 8.6  XBee Pin Descriptions and Functions



                                What is even more interesting than the hardware design is the data transmission and
                             reception protocol that XBee implements, which I will discuss in the next section.


                        XBee Data Protocol
                             The XBee uses a highly capable networking protocol named ZigBee, which is also called a
                             Personal Area Network (PAN). I will endeavor to keep the technical jargon to a minimum;
                             however, it is important that you get at least a fundamental knowledge of how the ZigBee
                             network functions in case something does not work as planned.
                                Zigbee was designed to be compliant with the ISO 7 Layer network model. As such, its
                             inherent design is based upon proven computer network concepts that are robust, efficient,
                             and well understood by most system designers. Figure 8.19 shows the ZigBee logical
                             network stack with the corresponding ISO layer number. All subsequent network software
                             developed for the ZigBee network follows this model.
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