Transmit Data Flow Control (&Hn); When It Is Required; Hardware Control; Software Control - US Robotics COURIER User Manual

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COURIER HIGH SPEED MODEMS
TRANSMIT DATA FLOW CONTROL (&Hn)
This type of flow control is for data transmitted to the modern by
its attached DTE, that is, by your computer or terminal. The
modern monitors its buffer as data comes from the DTE. If the
buffer approaches 90% capacity, the modem signals the DTE to
stop transmitting. When the modern has sent enough data over
the link to half empty the buffer, it signals the DTE to resume
transmitting.
When it is Required
Transmit Data flow control should be enabled in the following
situations.
• You're using error control (any allowable speed above
300 bps), with or without data compression.
• The DTE rate is higher than the link rate, for example,
the DTE is sending data to the modern at 38.4K or 19.2K
bps and the link rate is 14.4K bps. Another example is a
DTE rate of 19.2K bps and a link rate of 2400 bps.
NOTE: The modern uses either hardware or software flow
control. Your software and machine must support whichever
type you select, although we recommend hardware flow control,
if possible. Review the quick-configuration guide that came
with the modern and, if necessary, your software and computer
manuals.
Hardware Control
The modern drops the Clear to Send (CTS) signal it's been
sending to the DTE when the modem's buffer nears 90%
capacity. It starts sending CTS again when the buffer is about
half full.
Software Control
The modern sends the DTE 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
XOFF
<Ctrl>-Q
<Ctrl>-S
(ASCII 17 Decimal, 11 Hex)
(ASCII 19 Decimal, 13 Hex)
Interface Controls 4- 7