Keep It Simple Stupid -- KISS

KISS mode is a feature of some TNCs, and the only mode of other TNCs. The basic idea is that TNCs were originally designed for a person to operate, with readable commands and responses. The TNC made it practical for a person to get on the air on packet and hold conversations with multiple people.

The problem with a TNC's human-usable interface is that it is hard to operate from the perspective of a computer. Since everybody who uses packet these days uses a computer to drive it, having all sorts of readable control capability is a disadvantage. KISS mode is a command interface that lets the computer do most of the work, and makes it possible for the computer to have very fine control of what the TNC is doing.

Examples of where KISS is easier for the computer:
In human mode the TNC could be ready to accept a command and would tell you so with the cmd: prompt. Typing K puts the TNC back into the condition where it will send what is typed. Sending control-C will go back to command mode. However, if an incoming message comes in, it gets displayed (or sent to the computer) regardless of whether the TNC is waiting for a command or not.
In KISS mode the computer precedes a command with a special character. Every time the TNC sends something to the computer, the TNC precedes the message with special characters to indicate what the message means.

Another example
In human mode the user can tell the TNC to connect to a station, and then can switch streams and then connect to another station, and so on. The TNC will send everything that is typed to whatever station is on the stream currently selected. When an incoming packet arrives the TNC will send notice that the stream has changed, but only send that message each time the stream changes, not for each message.
In KISS mode every message that the TNC puts out to the computer is preceded by special characters that indicate what stream the message is from. Every message that is sent by the computer is preceded by special characters that tell the TNC what to do with the message.

Switching in and out of KISS mode

On a TNC having KISS mode as an option, there is a command provided to let a person put the TNC into KISS mode. Once the TNC is in KISS mode it is not likely to drop back to normal command mode because if it was easy, it could be done by mistake when the computer is trying to control the TNC with KISS commands.

If you put the TNC in KISS mode it may erase all configurations and saved mail, if your TNC has a mailbox.

When KISS mode was first made available, it came in the form of EPROMs that replaced the factory firmware in the TNC. There was no getting in and out of KISS mode.

Modern TNCs may come with only KISS mode, since there is an expectation that you are using a computer to control it anyway.

How do I get out?

You can take the TNC out of KISS mode but you need a few pieces of knowledge and hardware. First you need to know the serial baud rate of the TNC and that it is actually in KISS mode. Second you need a PC with a terminal program connected to the TNC. If you have these things, then you can use the extended keyboard sequences on a typical PC keyboard to send the not-so-secret code to the TNC.
Hold down the ALT key and on the numeric keypad press 192, then release the ALT key.
Do the same for 255, and then for 192 again.
That should do it.

If the keypad method didn't work or you don't know the baud rate, you can always do a hard reset on the CPU of the TNC. How you do that is up to the TNC manufacturar but in general you have to power down the TNC, open it up, remove the battery or remove the battery jumper, allowing the TNC memory to decay out. That could take a few minutes.
Then re-connect the battery, and power up the TNC.