User's Manual

©2009DigiInternational,Inc. 17
2.RFModuleOperation
Serial Communications
The XBee RF Modules interface to a host device through a logic-level asynchronous serial port.
Through its serial port, the module can communicate with any logic and voltage compatible UART;
or through a level translator to any serial device (for example: through a RS-232 or USB interface
board).
UART Data Flow
Devices that have a UART interface can connect directly to the pins of the RF module as shown in
the figure below.
SystemDataFlowDiagraminaUART‐interfacedenvironment
(Low‐assertedsignalsdistinguishedwithhorizontallineoversignalname.)
Serial Data
Data enters the module UART through the DIN (pin 3) as an asynchronous serial signal. The signal
should idle high when no data is being transmitted.
Each data byte consists of a start bit (low), 8 data bits (least significant bit first) and a stop bit
(high). The following figure illustrates the serial bit pattern of data passing through the module.
UARTdatapacket0x1F(decimalnumberʺ31ʺ)astransmittedthroughtheRFmodule
ExampleDataFormatis8‐N‐1(bits‐parity‐#ofstopbits)
Serial communications depend on the two UARTs (the microcontroller's and the RF module's) to be
configured with compatible settings (baud rate, parity, start bits, stop bits, data bits).
The UART baud rate, parity, and stop bits settings on the XBee module can be configured with the
BD, NB, and SB commands respectively. See the command table in chapter 10 for details.
Serial Buffers
The XBee modules maintain small buffers to collect received serial and RF data, which is illustrated
in the figure below. The serial receive buffer collects incoming serial characters and holds them
until they can be processed. The serial transmit buffer collects data that is received via the RF link
that will be transmitted out the UART.
DIN (data in)
DIN (data in)
DOUT (data out)
DOUT (data out)