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.