KISS TNC

The KISS TNC app turns the PortaPack into a bidirectional KISS TNC (Terminal Node Controller) over the USB serial port. It receives APRS/AX.25 packets from radio and forwards them to a connected PC as KISS frames, and transmits KISS frames received from the PC over radio as AFSK 1200 baud AX.25.

What is KISS TNC?

KISS (Keep It Simple, Stupid) is a protocol that allows a PC application to send and receive AX.25 packet radio frames through a TNC over a serial connection. With this app, the PortaPack acts as the TNC, and the USB serial port is the link to the host PC. Any APRS or packet radio software that supports a serial KISS TNC can be used — for example Direwolf (as a client), APRSISCE/32, APRSdroid, or custom scripts.

Controls

Field	Description
Frequency	RX/TX frequency. Default: 144.390 MHz (APRS standard)
RF Amp	RF amplifier on/off
LNA	LNA gain
VGA	VGA gain
RSSI	Signal strength indicator
USB	Shows Connected or Disconnected — whether the PC has opened the USB serial port
RX	Count of AX.25 packets received from radio and forwarded to the PC
TX	Count of frames received from the PC and transmitted over radio
Console	Live log of received packets, shown as SOURCE>DESTINATION

The status line at the top toggles between Listening (RX active) and Transmitting (TX in progress).

Connecting a PC

Connect the PortaPack to the PC via USB. The USB serial port appears as a standard CDC serial device. Open it in your packet radio software and configure it as a KISS TNC at any baud rate (the USB CDC connection is not baud-rate limited).

Note

On Linux the device typically appears as /dev/ttyACM0. On Windows it appears as a COM port. No driver installation is needed on Linux or macOS; Windows may require the standard CDC driver.

Operation

• RX: The app continuously listens on the configured frequency. When a valid AX.25 packet is decoded, it is wrapped in a KISS frame and sent to the PC over USB serial. The RX counter increments and the packet's source and destination callsigns appear in the console.
• TX: When the PC sends a KISS data frame (command byte 0x00) over USB serial, the app switches to transmit mode, sends the AX.25 frame as AFSK 1200 baud Bell 202, then automatically returns to RX mode. The TX counter increments for each transmitted frame.

Warning

The maximum transmittable frame payload is 200 bytes. Frames larger than this are silently discarded.

Technical details

Parameter	Value
Modulation	AFSK Bell 202
Baud rate	1200 baud
Mark / Space	1200 Hz / 2200 Hz
Default frequency	144.390 MHz
Max TX frame size	200 bytes
KISS framing	Standard KISS (0xC0 FEND, 0xDB FESC escaping)