User Manual

XBee®/XBee‐PRO®RFModules‐802.15.4‐v1.xEx[2011.06.1]
©2011DigiInternatonal,Inc. 14
Flow Control
Figure2‐03. InternalDataFlowDiagram
DI (Data In) Buffer
When serial data enters the RF module through the DI pin (pin 3), the data is stored in the DI Buf-
fer until it can be processed.
Hardware Flow Control (CTS
). When the DI buffer is 17 bytes away from being full; by default,
the module de-asserts CTS
(high) to signal to the host device to stop sending data [refer to D7
(DIO7 Configuration) parameter]. CTS
is re-asserted after the DI Buffer has 34 bytes of memory
available.
How to eliminate the need for flow control:
Case in which the DI Buffer may become full and possibly overflow:
Refer to the RO (Packetization Timeout), BD (Interface Data Rate) and D7 (DIO7 Configuration) com-
mand descriptions for more information.
DO (Data Out) Buffer
When RF data is received, the data enters the DO buffer and is sent out the serial port to a host
device. Once the DO Buffer reaches capacity, any additional incoming RF data is lost.
Hardware Flow Control (RTS
). If RTS is enabled for flow control (D6 (DIO6 Configuration)
Parameter = 1), data will not be sent out the DO Buffer as long as RTS
(pin 16) is de-asserted.
Two cases in which the DO Buffer may become full and possibly overflow:
Refer to the D6 (DIO6 Configuration) command description for more information.
1. Send messages that are smaller than the DI buffer size (202 bytes).
2. Interface at a lower baud rate [BD (Interface Data Rate) parameter] than the throughput
data rate.
If the module is receiving a continuous stream of RF data, any serial data that arrives on the DI
pin is placed in the DI Buffer. The data in the DI buffer will be transmitted over-the-air when the
module is no longer receiving RF data in the network.
1. If the RF data rate is set higher than the interface data rate of the module, the module will
receive data from the transmitting module faster than it can send the data to the host.
2. If the host does not allow the module to transmit data out from the DO buffer because of
being held off by hardware or software flow control.