Serial Receive Buffer; Serial Transmit Buffer; Flow Control - Digi XBee3 DigiMesh 2.4 User Manual

Serial communication

Serial receive buffer

When serial data enters the XBee3 DigiMesh RF Module through the serial port, the device stores the
data in the serial receive buffer until it can be processed. Under certain conditions, the device may
receive data when the serial receive buffer is already full. In that case, the device discards the data.
The serial receive buffer becomes full when data is streaming into the serial port faster than it can be
processed and sent over the air (OTA). While the speed of receiving the data on the serial port can be
much faster than the speed of transmitting data for a short period, sustained operation in that mode
causes the device to drop data due to running out of places to put the data. Some things that may
delay over the air transmissions are address discovery, route discovery, and retransmissions.
Processing received RF data can also take away time and resources for processing incoming serial
data.
If the UART is the serial port and you enable the CTS flow control, the device alerts the external data
source when the receive buffer is almost full. The host delays sending data to the device until the
module asserts CTS again, allowing more data to come in.

Serial transmit buffer

When the device receives RF data, it moves the data into the serial transmit buffer and sends it out
the UART. If the serial transmit buffer becomes full and the system buffers are also full, then it drops
the entire RF data packet. Whenever the device receives data faster than it can process and transmit
the data out the serial port, there is a potential of dropping data.

Flow control

The XBee3 DigiMesh RF Module maintains buffers to collect serial and RF data that it receives. The
serial receive buffer collects incoming serial characters and holds them until the device can process
them. The serial transmit buffer collects the data it receives via the RF link until it transmits that data
out the serial port. The following figure shows the process of device buffers collecting received serial
data.
Use D6 (DIO6/RTS)
Digi XBee3® DigiMesh RF Module User Guide
and D7 (DIO7/CTS) to set flow control.
Serial receive buffer
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