EPS32 KISS TNC

There are many KISS TNCs available online, both commercially and as DIY projects. While most DIY options offer high-quality hardware, they often lack one critical component—open-source firmware. Many projects provide only the HEX file for pre-programmed microcontrollers, limiting the ability to modify or understand the underlying code.

This led me to embark on my own KISS TNC project, inspired by the work of KV4P. I replicated his audio interface for APRS, but when it came to the firmware, I wanted a fully customizable solution.

To kickstart the development, I experimented with AI-generated code. I provided OpenAI with my project details, including the ESP32 pinout, and asked for a simple KISS TNC implementation. The initial results were promising but required further refinement. Through multiple iterations, I improved the code to better align with the project’s needs.

Circuit:

To build KISS TNC, you’ll need a few essential components. This project is based on the ESP32 WROOM Dev Kit v4 and a simple audio interface for APRS. Here’s what you’ll need:

Components

1X ESP32 WROOM Devkit V4

2X 10nF Capacitor

1X 1K Resistor

3X 10K Resistor

1X BC547 or 2N2222 NPN Transistor

1X I/O Plug (minimum 4Pin)

You Can download Gerber files for Git Hub below:

PCB (Top/Bottom):

Code:

// https://uhpowerup.com/ //
//AI genrated code modified by 2E0UMR//
// use at your own risk //

#include <Arduino.h>
#include “BluetoothSerial.h”

#define ATR 25
#define AFR 34
#define PTT 4

BluetoothSerial BTSerial; // Bluetooth KISS Interface
bool useBluetooth = false;

// KISS protocol special characters
#define FEND 0xC0 // Frame End
#define FESC 0xDB // Frame Escape
#define TFEND 0xDC // Transposed FEND
#define TFESC 0xDD // Transposed FESC

void setup() {
Serial.begin(115200); // USB Serial KISS
BTSerial.begin(“ESP32_KISS_TNC”); // Bluetooth KISS (change”ESP32_KISS_TNC” to rename bluetooth)
pinMode(PTT, OUTPUT);
digitalWrite(PTT, LOW);

Serial.println(“ESP32 KISS TNC Ready”);
}

// Function to send KISS packets over serial/Bluetooth
void sendKISSPacket(uint8_t *data, size_t len) {
Serial.write(FEND);
BTSerial.write(FEND);

for (size_t i = 0; i < len; i++) {
if (data[i] == FEND) {
Serial.write(FESC);
Serial.write(TFEND);
BTSerial.write(FESC);
BTSerial.write(TFEND);
} else if (data[i] == FESC) {
Serial.write(FESC);
Serial.write(TFESC);
BTSerial.write(FESC);
BTSerial.write(TFESC);
} else {
Serial.write(data[i]);
BTSerial.write(data[i]);
}
}
Serial.write(FEND);
BTSerial.write(FEND);
}

// Function to generate AFSK tones (simplified example)
void transmitAFSK(uint8_t *packet, size_t len) {
digitalWrite(PTT, HIGH);
delay(50); // Small delay for radio keying

for (size_t i = 0; i < len; i++) {
// Generate 1200 Hz or 2200 Hz based on packet bits
tone(ATR, (packet[i] & 1) ? 2200 : 1200, 10);
delay(10);
}

delay(50); // Hold PTT briefly
digitalWrite(PTT, LOW);
}

// Function to decode received AFSK audio (simplified)
void receiveAFSK() {
int sample = analogRead(AFR);
if (sample > 2000) { // Threshold for detecting signal
uint8_t receivedPacket[] = { 0x82, 0xA6, 0x40, 0x61, 0xE0, 0x03, 0xF0, ‘H’, ‘e’, ‘l’, ‘l’, ‘o’ };
sendKISSPacket(receivedPacket, sizeof(receivedPacket));
}
}

void loop() {
// Check USB Serial for KISS frames
if (Serial.available()) {
uint8_t kissFrame[256];
size_t len = Serial.readBytes(kissFrame, sizeof(kissFrame));
transmitAFSK(kissFrame, len);
}

// Check Bluetooth Serial for KISS frames
if (BTSerial.available()) {
uint8_t kissFrame[256];
size_t len = BTSerial.readBytes(kissFrame, sizeof(kissFrame));
transmitAFSK(kissFrame, len);
}

receiveAFSK(); // Process incoming audio
}

Images:

TNC setup (Simplified):

TNC will show up as COM port with 115200 Baud rate via USB or Bluetooth (Default name”ESP32_KISS_TNC”)

I have tested it on YAAC. Unfortunately VHF have very poor Propagation on ground level at my QTH (IO91VL) so unable to connect to any APRS RF gates but RF to RF I was able to send and receive Packets from one HT(KGuv9d) to N7500.

I will update further after testing it from another location where VHF propagation is good.

73’s

2E0UMR