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2. RF Module Operation
Serial Communications
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The XBee-PRO®OEM 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 Digi
proprietary 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.)
CMOS Logic (3.0-3.6V)
DIN (data in)
CTS
XBee
Microcontroller
DOUT (data out)
Module
RTS
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)
The module UART performs tasks, such as timing and parity checking, that are needed for data
communications. Serial communications depend on the two UARTs to be configured with
compatible settings (baud rate, parity, start bits, stop bits, data bits).
Serial Buffers
The XBee-PRO modules maintain 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.
© 2008 Digi International, Inc.
CMOS Logic (3.0-3.6V)
DIN (data in)
CTS
XBee
Microcontroller
DOUT (data out)
Module
RTS
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