TNC Operating Modes Technical Seminar – Sept. 26/98

By Lane, VE7IHL

What are TNC Operating Modes?

Each TNC (Terminal Node Controller) can operate in several different modes of operation. This refers to how the TNC communicates on the RS-232 serial interface cable to your computer, and how its firmware operates differently. The default mode is known as Terminal mode, with most TNCs also supporting a KISS mode, some a proprietary HOST mode, and other various modes as provided by different TNC manufacturers.

What I am NOT discussing here, are additional TNC firmware features that can operate in conjunction with the normal TNC's operation, such as PBBS (Personal Mail Box), and on some TNCs a NODE Switch feature (KA-Node for example).

Terminal Mode

This is the most familiar mode, and is called Terminal mode. This is the mode that the TNC is run in when powered up for the first time, and what is used with either TPK or the Winpack software packages. (As well as may other packet terminal programs) This mode allows you to communicate with your TNC using a 'dumb' RS-232 serial terminal, or a terminal emulation software program running on your PC. The TNC performs all of the AX.25 protocol itself.

This mode allows you to communicate with your TNC with "English-like" command strings. It is easiest for humans to understand. (But not as easy for a computer program to understand) It is also referred to as a simple ASCII mode of communication.

Special Note: The Kantronics TNCs, when operating in their factory default Terminal mode, are running in what they call New User mode. This is basically the same as normal Terminal mode, but restricts the number of commands that the user can enter. This can be very annoying when you try to enter some 'normal' TNC commands they don't work. To get into normal Terminal mode, enter the command 'INT TERMINAL', then 'RESET'.

Host Mode

The Host mode of operation is a proprietary mode that allows computer programmers to write TNC communication programs easier. Each manufacturer's Host mode is different from other manufacturers. This forces programmer's to either produce different programs for different TNCs, or to provide different 'TNC driver' support for their programs.

This mode of communication is a binary mode, and thus a normal terminal program would not be able to be used in this mode. The TNC performs all of the AX.25 protocol itself.

 KISS Mode

The KISS mode of operation is used when the normal AX.25 communication protocol that is built into the TNC's firmware, is essentially bypassed. Why would this be done? It is done when a more advanced communication protocol is to be used (such as Tcp/Ip), or because of limitations of the built in AX.25 firmware. (Such as the number of incoming users supported is too small). Many BBS programs, the BPQ node switch software, the Linux operating system, and others use this mode. This is also a binary mode of communication.

When the KISS mode is used, the TNC essentially turns into a simple modem device, which only concerns itself with transmitting and receiving raw HDLC packets. It no longer performs the normal AX.25 protocol. The host computer's software MUST now do this. The TNC still maintains control of the timing of when it transmits packets, using the normal TxDelay, and Persist TNC parameters. This allows the TNC to attempt to minimize collisions on the RF channel.

Other Modes

There are some other unique operating modes, each with their own type of serial port communication protocol and functional operation, that is provided by different manufacturer's TNCs. The following is a short list of some of these other operating modes.

Kantronics KPC-3 (Plus) TNC

How to Switch Modes

Mode: Kantronics TNCs: AEA TNCs: PacComm TNCs:

to KISS INT KISS, RESET KISS ON KISS ON, RESTART

out of KISS C0,FF,C0 sequence (3) ^C's C0,FF,C0 sequence

to TERMINAL INT TERMINAL, RESET remove battery remove battery jumper

to HOST INT HOST, RESET HOST ON

out of HOST C0,Q,C0 sequence (3) ^C's

to DAMA DAMA ON

out of DAMA DAMA OFF

to MODEM INT MODEM, RESET

out of MODEM (3) ^C's

to GPS INT GPS, RESET GPS ON

out of GPS (3) ^C's \

Want More Information?

Read your TNC's reference manual. They are filled with lots of information. All of the material for this technical seminar was obtained from the standard manuals that are supplied by each TNC manufacturer.