Functions
INSYS Modem 336 LL 2D/4D 5.x Originate / Answer
12.3
Data Flow Control (Handshake)
The data flow control ensures that the data transfer is interrupted as soon as the modem
buffer exceeds a certain level. Two data flow control options are available: Via the con-
trol lines RTS and CTS, or via the control characters XON/XOFF which are inserted into
the data stream.
12.3.1
Hardware data flow control (RTS/CTS)
The hardware data flow control works in two directions. When the critical buffer level is
exceeded, the modem will set the CTS line to "low" and will thus indicate to the applica-
tion to interrupt the dataflow. When the buffer is emptied sufficiently for the INSYS Mo-
dem 336 LL 2D/4D 5.x Originate / Answer to be able to receive data again, the CTS line is
set to "high". Reversely, the application can also indicate to the INSYS Modem 336 LL
2D/4D 5.x Originate / Answer to interrupt the data flow. This is done via the RTS line. If it
is set to "low", the modem will interrupt the data flow to the application. The application
will set it to "high" to request data from the INSYS Modem 336 LL 2D/4D 5.x Originate /
Answer.
The data flow control with RTS/CTS behavior is active by default.
Configuration with AT commands
To switch the data flow control on and to set
the type to RTS/CTS, use:
AT&K3
To switch the data flow control off, use:
AT&K0
12.3.2
Software data flow control (XON/XOFF)
When the input buffer of the modem exceeds a certain fill state, the modem will insert
an XOFF character into the data stream to the application. This character will cause the
application to send no more data. It will depend on the according application software if
the XON/XOFF data flow control is supported.
After the input buffer of the modem is emptied so much that data can be received again,
the modem will send an XON character to the application. This character will cause the
application to send data to the modem again. Analogously, the application can insert
XON/XOFF characters into the data stream to switch the data flow on and off. The
XON/XOFF data flow control is only available when the transmitted data do not contain
the characters XON or XOFF, which usually appear only in actual ASCII texts (7 bit). When
binary data (programs, etc.) are transmitted, or in the XMODEM transmission protocol,
for example, occasionally appearing XON or XOFF characters would disturb the opera-
tion.
Configuration with AT commands
To switch the data flow control on and to set
the type to XON/XOFF, use:
AT&K4
To switch the data flow control off, use:
AT&K0
34
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