Serial flow control is used to prevent buffer overruns. When a host microprocessor is shifting data into the radio modem''''s serial buffer, if the buffer is getting full the radio will de-assert Clear-To-Send (CTS) signal indicating to the host that it is no longer clear to send more characters. The host should then stop the transfer of data until the CTS signal asserts.
All Digi radio modems / modules support CTS hardware flow control. And all Digi radio modems have at least 100 byte buffers so it will not always be necessary to observe the hardware flow control.
Consider the following factors to determine whether it is necessary to observe flow control:
Does the protocol ever send more that 100 bytes before pausing to wait for a response?
Are retries enabled and/or is the serial interface baud rate faster than the radio''''s throughput baud rate?
If you answered YES to both of the above questions, then it is likely that it will be necessary to observe flow control to avoid buffer overruns.
If observing hardware flow control is not an option then consider using software flow control. Software flow control is only an option when transfering ASCII data because is uses control characters embedded in the data stream to signal when to start and stop the transmission. (0x11=XON and 0x13=XOFF)
Also note that the XStream radio modem can observe RTS flow control if the host needs to stop the data flow out of the radio modem (ATRT2). The XCite radio modem can not.