© 2014 Digi International Inc. 19
XBee/XBee-PRO
®
DigiMesh 2.4 User Manual
Process of XBee-PRO buffers collecting received serial data
Serial Receive Buffer
When serial data enters the RF module through the DIN Pin (pin 3), the data is stored in the
serial receive buffer until it can be processed. Under certain conditions, the module may not be
able to process data in the serial receive buffer immediately. If large amounts of serial data are
sent to the module, CTS flow control may be required to avoid overflowing the serial receive
buffer.
Cases in which the serial receive buffer may become full and possibly overflow:
1 If the module is receiving a continuous stream of RF data, the data in the serial receive buffer
will not be transmitted until the module is no longer receiving RF data.
2 For mesh networking firmware, if the module is transmitting an RF data packet, the module
may need to discover the destination address or establish a route to the destination. After
transmitting the data, the module may need to retransmit the data if an acknowledgment is
not received, or if the transmission is a broadcast. As a result, these issues could delay the
processing of data in the serial receive buffer.
Serial Transmit Buffer
When RF data is received, the data is moved into the serial transmit buffer and is sent out the
serial port. If the serial transmit buffer becomes full enough such that all data in a received RF
packet won’t fit in the serial transmit buffer, the entire RF data packet is dropped.
Cases in which the serial transmit buffer may become full resulting in dropped RF packets
1 If the RF data rate is set higher than the interface data rate of the module, the module could
receive data faster than it can send the data to the host. Even occasional transmissions from
a large number of modules can quickly add up and overflow the transmit buffer.
2 If the host does not allow the module to transmit data out from the serial transmit buffer
because of being held off by hardware flow control.
Serial
Receiver
Buffer
RF TX
Buffer
Transmitter
RF Switch
Antenna
Port
Receiver
Serial Transmit
Buffer
RF RX
Buffer
Processor
GND
DIN
VCC
DOUT
CTS
RTS
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