COURIER HIGH SPEED MODEMS
Data Mode Operations 4-25
Hardware Control
The modem drops the Clear to Send (CTS) signal it's been
sending to the computer or terminal when the modem's buffer
nears 90% capacity. It starts sending CTS again when the buffer
is about half full.
Software Control
The modem sends the computer or terminal the standard ASCII
Transmit OFF (XOFF) character, <Ctrl>-S, when its buffer nears
90% capacity. The modem sends the ASCII Transmit ON
character, <Ctrl>-Q, when the buffer is about half full. ASCII
definitions are as follows:
XON <Ctrl>-Q (ASCII 17 Decimal, 11 Hex)
XOFF <Ctrl>-S (ASCII 19 Decimal, 13 Hex)
NOTE: You should set your software as well to either
hardware or software flow control. Some programs also require
that you turn off the type you are not using.
The ASCII characters may be user-defined. See Registers S22
and S23 in Appendix B. That appendix also includes an ASCII
chart.
WARNING: If possible, always use hardware flow control, the
factory default. You may lose data if XON/XOFF (<Ctrl>-S,
<Ctrl>-Q) characters occur in the data stream from other
sources. They may, for example, come from the remote system:
an XON from the remote system, after your modem has sent an
XOFF, can result in buffer overflow.
<Ctrl>-S (XOFF) and <Ctrl>-Q (XON) characters also occur in
binary files, and are used by Xmodem-type protocols. You risk
having these characters misinterpreted as modem flow control
characters and dropped from the data stream.
If you cannot use hardware flow control and if you're
transferring non-text (binary) files, or using an Xmodem-type
protocol, disable flow control entirely (&H0). In addition, be
sure the modem is set to &B0 and &N0, so that the serial port
and connection rates are equal.