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2. RF Module Operation
2.1. Serial Communications
2.1.1. UART Data Flow
2.1.2. Serial Buffers
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The XBee Series 2 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
MaxStream proprietary RS-232 or USB interface board).
Devices that have a UART interface can connect directly to the pins of the RF module as shown in
the figure below.
Figure 2‐01. System Data Flow Diagram in a UART‐interfaced environment
(Low‐asserted signals distinguished with horizontal line over signal name.)
DIN (data in)
DOUT (data out)
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.
Figure 2‐02. 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).
The XBee Series 2 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.
© 2007 Digi International, Inc.
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DIN (data in)
DOUT (data out)
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