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11-2
C
HAPTER
11: F
LOW
C
ONTROL
Hardware and
Software Flow
Control
There are two kinds of flow control: hardware and software. Business
Modems support both, but your computer and communications software
must also support the kind of flow control you choose.
Hardware Flow
Control
Business Modems implement hardware flow control by detecting that a
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
Control
Business Modems implement software flow control by detecting that a
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.