Terminal Node Controller (TNC)

The traditional TNC is a standalone hardware device, which connects to both the radio and the station computer. These implement the entire AX.25 packet protocol, and provide additional features such as an internal mailbox, visible indicator lights, command mode customization, and built-in help. SCCo ARES/RACES packet training focuses on this type of TNC. It is the type used in the county EOC, most city EOCs and hospitals, etc. It is preferred for any station likely to be used by operators other than the owner.

Some newer “KISS Mode” TNCs implement only part of the AX.25 protocol, leaving the rest to be handled by software on the packet computer. Some newer radios have TNCs built in, with varying levels of functionality. And there are “software TNCs” available that implement the entire AX.25 stack on the station computer, connecting directly to the radio without any hardware TNC at all. These variant TNC types are also effective, and can be used in personal stations or when no full-function hardware TNC is available. However, SCCo training does not cover them, they sometimes lack useful features, and they can be finicky to configure and troubleshoot. They should be avoided on stations that will be used by multiple operators.

For software TNCs, using an external soundcard (usually connected with USB) can avoid most problems with PC operating system sounds going to the radio or with different applications adjusting the audio levels. USB can be susceptible to RF interference, so keep the cable coiled up short and use ferrite beads.

The linked document describes required and recommended configuration settings for a variety of TNC models.

All TNCs (except radio-integrated TNCs) must be tuned for the audio output level of the radio they’re connected to. Whenever setting up a new station, swapping TNCs, or swapping radios, make sure to tune the FM deviation on the TNC appropriately. The process for this varies based on the TNC model: some can be tuned with a software command, while others require a hardware adjustment.

If your computer does not have a female DB-9 port, we recommend having a USB-to-serial adapter in your kit. Even if your own TNC doesn’t need one, you may need it if you have to swap out the TNC during a deployment.

The TNC connects to the radio with a cable that carries audio, squelch, and PTT signals and is specific to the particular TNC/radio combination. Commercially made cables are available for many combinations. Many Kenwood and Yaesu mobile radios use a 6-pin mini-DIN connector with a common pin-out. The Alinco radios use a DB-9 connector (which is *not* a serial port despite looking like one). Consult the TNC and radio manuals for the proper pinouts on each end of the cable.

All cables act as antennas, and currents induced RF signals can cause misbehavior. This is especially true if the radio antenna is nearby. Coil up excess cable to reduce its effectiveness as an antenna. Place ferrite beads at each end of the cable. Avoid running any cable alongside the antenna feed cable.