US Robotics COURIER HST User'smanual page 32

Transmit Data
Flow Control
(&Hn)
COURIER HST
terminal will back up and perhaps overflow the
modem's buffer.
• The rate at the terminal interface differs from the link
rate, e.g. , the terminal's sending at 19. 2k bps and
the link rate is 9600 bps. This setup offers the great-
est throughput, but the modem requires the ability
to signal the terminal when the modem's buffer is
reaching capacity.
• The volume of data transfer is high.
The modem monitors its buffer as data comes from the ter-
minal. If the buffer approaches 90o/c capacity, the modem
signals the terminal to stop sending. When the modem has
sent enough data over the link to half empty the buffer, it
signals the terminal to resume transmitting.
Two types of signals are used:
• Hardware: the modem raises or lowers the Clear To
Send (CTS) signal via Pin 5 on the RS-232C interface.
• Software: the modem sends the conventional ASCII
Transmit on/off (XON/XOFF) characters, as follows:
XON
XOFF
<Ctrl>-Q
<Ctrl>-S
(ASCII 17 Decimal, 11 Hex)
(ASCII 19 Decimal, 13 Hex)
The ASCII characters may be user-defined; see
S-registers S22 and S23 in Appendix B-5 and the
ASCII chart in Appendix B-6 or on the Quick-
Reference card.
'.\CHE:
If possible, use hardware control as it is more
efficient and doesn ' t affect the data stream.
&HO
Flow control of transmitted data is disabled. This setting is
the default.
&HT
Use hardware flow control. If your terminal supports Clear
To Send (CTS. RS-232C Pin 5), the Courier lowers CTS
when the buffer nears 90o/c capacity, and raises CTS again
when the buffer is about half full.
&H2
Use software flow control. The Courier sends the terminal
the XON/XOFF characters to control the input of data to
the buffer.
&H3
Use both hardware and software flow control. If you
are unsure about what your equipment supports, select
this option.
4-3