The KPC-3 is a Terminal Node Controller (TNC). A TNC is very
similar to a telephone modem in that it receives digital
signals from your computer (Terminal) and converts them to
tones suitable for transmission to a distant location. The
TNC also receives tones from your radio and converts them
into the digital signals understood by your computer.
A TNC, however, does much more, because it also controls the
push-to-talk line of your transmitter, keying the radio
whenever it needs to send data. It also converts the data
you want to send into a “packet”, adding the required
addressing, error checking and control information to insure
the data gets from one Node to the next. The error checking
implemented in your TNC must be the same as the error
checking used by any other station you want to talk to, and
this standard method is called a protocol. The protocol used
in Amateur Radio Packet TNCs is called AX.25. Different
protocols are used for other modes of operation, such as AMTOR.
In order for your TNC to do something, you must issue
instructions to it, letting it know exactly what you want
done. In order to accomplish this, the TNC must be in the
Command Mode (expecting you to give it instructions) and any
time you want to change the way your TNC operates, you must
be in this mode. The TNC tells you that it is ready for your
commands by sending you the prompt **cmd:”.
When you want to send data to another station, you must
place your TNC into the CONVERS mode, which allows you to
converse with other users. This is normally done
automatically for you when you connect to another station,
or when another station connects to you.