3 KISS TNC
Speedster04 edited this page 2026-03-24 20:27:40 +01:00

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)