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12-2
C
12: F
HAPTER
LOW CONTROL
Hardware and
Software Flow
Control
Hardware Flow
Control
Software Flow
Control
There are two kinds of flow control: hardware and software. Courier
V.Everything modems support both, but your computer and
communications software must also support the kind of flow control you
choose.
Courier V.Everything 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 sends CTS to restart the flow of data.
Courier V.Everything 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 Courier V.Everything 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 modem t
recognise 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 A, S-Registers.
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