3Com 56K manual Hardware, Software Flow, Control

11-2CHAPTER 11: FLOW CONTROL

Hardware and	There are two kinds of flow control: hardware and software. Business
Software Flow	Modems support both, but your computer and communications software
Control	must also support the kind of flow control you choose.
Hardware Flow	Business Modems implement hardware flow control by detecting that a
Control	buffer is 90% full and then interrupting the Clear to Send (CTS) signal to
	stop the flow of data. When the buffer drops back to 50% full, the
	Business Modem sends CTS to restart the flow of data.
Software Flow	Business Modems implement software flow control by detecting that a
Control	buffer is 90% full and then sending special characters in the data stream
	to stop the flow of data. When the buffer drops back to 50% full, the
	Business Modem sends special characters in the data stream to restart the
	flow of data.
	The problem with software flow control is that the characters used to
	stop (<Ctrl>Q) and start (<Ctrl>S) the flow of data can occur naturally in
	the data flow. Enabling software flow control instructs the Business
	Modem to recognize and act on these characters, even if they are not
	intended to control the data flow.
	Using software flow control may prove satisfactory if you're transferring
	text files only.
	The start command is called XON (for transmit on) and the stop
	command is called XOFF (transmit off). You can change the characters
	used. See Registers S22 and S23 in Appendix B, Alphabetic Command
	Summary.