in the sending TNC; if no response was received, and the RELINK
parameter is ON, the TNC at station A would then begin to issue connect re
-
quests to station B since there is still an outstanding packet of information. This
is the major difference between version 1 and version 2. The connect attempts
would then continue for the number of retries set in the TNC, and if no response
was received from station B after all of the above, station A would disconnect
and discard the packet. The parameter RELINK is defaulted OFF to avoid the re
-
connect attempt.
Flow Control
The flow control commands insure that the TNC gets everything that is sent to it
by the computer and that the computer gets everything the TNC sends it. When
the computer sends the TNC data, the TNC stores this data in a buffer until it can
packetize it, send it, and get acknowledgments. Similarly, when the TNC sends
the computer data, the computer stores the data in a buffer until it can be pro-
cessed, stored to disk, sent to printer, or whatever.
This buffer area is of limited size; if more data is sent than will fit in the buffer
the extra data will be lost. To make sure each device gets all the data it should
from the other device, the two devices can tell each other to start and stop send-
ing data. This is called Flow Control and it can be accomplished in either of two
ways, via software or via hardware.
Which way you implement this depends on the capabilities of your computer
communications program and personal preference. The cable between your com
-
puter and TNC must also be wired appropriately.
Software Flow Control
Software flow control sends special characters on the TXD and RXD lines of the
RS-232 cable. These are the same lines used for sending regular data between the
TNC and computer. Software flow control normally sends a <Ctrl+S> to stop
data and a <Ctrl+Q> to restart data. When a buffer gets close to full, the device
will send a <Ctrl+S> and expect the other device to stop. When the buffer gets
nearly empty, it will send a <Ctrl+Q> to tell the other device to send more data.
How full or empty a buffer is when the special characters are sent is determined
by the program. But, since the regular data lines are being used, a <Ctrl+S> sent
Packet Modes of Operation Overview of Packet Communication
User’s Guide 154 KAM’98 v 8.3
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