Terminal node controller

It is similar in function to the Packet Assembler/Disassemblers used on X.25 networks, with the addition of a modem to convert baseband digital signals to audio tones.

These never gained much popularity because only a bare printed circuit board was made available and builders had to gather up a large number of components.

A typical model consists of a microprocessor, a modem, and software (in EPROM) that implements the AX.25 protocol and provides a command line interface to the user.

(Commonly, this software provides other functionality as well, such as a basic bulletin board system to receive messages while the operator is away.)

Data from the terminal is formatted into AX.25 packets and modulated into audio signals (in traditional applications) for transmission by the radio.

Since the late 1990s, most AX.25 usage has shifted to a different one-to-many communication paradigm with the Automatic Packet Reporting System (APRS).

As home computers made their way into ham "shacks," there was a movement toward simpler, cheaper "KISS" (Keep It Simple, Stupid) devices.

The importance of location to the APRS system has fueled development of a new generation of small low-power TNCs often integrated with a GPS module for use in mobile tracking stations.

Although typically described as data ports, the interface between a radio and a TNC is almost entirely analog audio (plus lines for PTT and squelch).