diff --git a/create_sinus_tables.py b/create_sinus_tables.py
index de2a6e2..aecddeb 100755
--- a/create_sinus_tables.py
+++ b/create_sinus_tables.py
@@ -52,10 +52,14 @@ void sendZero(uint32_t *us, uint8_t *phaseShift) {{
     while ((t = micros() - *us) < INV_BAUD) {{
         OUT_PORT = pgm_read_byte(&SINUS2200[ (PS2200[*phaseShift] + t) % N_SINUS_2200 ]);
     }}
-    (*phaseShift)++;
+    *phaseShift = (*phaseShift + 1) % N_SHIFTS;
     *us += INV_BAUD;
 }}
 
+void setZero() {{
+  OUT_PORT = pgm_read_byte(&SINUS2200[0]);
+}}
+
 ''')
 
 
diff --git a/include/afsk_sinus.hpp b/include/afsk_sinus.hpp
index b39f490..5b21486 100644
--- a/include/afsk_sinus.hpp
+++ b/include/afsk_sinus.hpp
@@ -27,7 +27,11 @@ void sendZero(uint32_t *us, uint8_t *phaseShift) {
     while ((t = micros() - *us) < INV_BAUD) {
         OUT_PORT = pgm_read_byte(&SINUS2200[ (PS2200[*phaseShift] + t) % N_SINUS_2200 ]);
     }
-    (*phaseShift)++;
+    *phaseShift = (*phaseShift + 1) % N_SHIFTS;
     *us += INV_BAUD;
 }
 
+void setZero() {
+  OUT_PORT = pgm_read_byte(&SINUS2200[0]);
+}
+
diff --git a/src/main.cpp b/src/main.cpp
index 84e3ab1..47af543 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1,18 +1,73 @@
 
 #include <Arduino.h>
+#include <util/crc16.h>
 
 #include "afsk_sinus.hpp"
 
+#define NCOMMENT 0
+#define CALLSIGN "HB3YBQ"
+
+
+uint8_t txdelay[32];
+
+struct ax25 {
+  uint8_t sflag;
+  uint8_t daddr[7];
+  uint8_t saddr[7];
+  uint8_t ctrl;
+  uint8_t proto;
+  struct {
+    uint8_t dtid;
+    uint8_t type;
+    char time[7];
+    char lat[8];
+    uint8_t sym_table_id;
+    char lon[9];
+    uint8_t sym_code;
+    char comment[NCOMMENT];
+  } __attribute__((packed)) aprs;
+  uint16_t fcs;
+  uint8_t eflag;
+} __attribute__((packed)) frame;
+
+enum AprsSymbol : uint8_t {
+  NONE,
+  AMBULANCE,
+  BUS,
+  FIRE_TRUCK,
+  BICYCLE,
+  YACHT,
+  HELICOPTER,
+  SMALL_AIRCRAFT,
+  SHIP,
+  CAR,
+  MOTORCYCLE,
+  BALLOON,
+  JEEP,
+  RECREATIONAL_VEHICLE,
+  TRUCK,
+  VAN
+};
+
 
 extern void sendOne(uint32_t *us, uint8_t *phaseShift);
 extern void sendZero(uint32_t *us, uint8_t *phaseShift);
+extern void setZero();
 
 uint32_t m = 0;
 uint8_t phaseShift = 0;
+bool nrzi = true;
 
-void sendBell202(uint8_t byt) {
+void sendBell202(uint8_t byt, bool sync) {
+  if (sync) {
+    phaseShift = 0;
+    m = micros();
+  }
   for (uint8_t i = 0; i < 8; ++i) {
-    if ((byt >> i) & 1) {
+    if (!((byt >> i) & 1)) {
+      nrzi = !nrzi;
+    }
+    if (nrzi) {
       sendOne(&m, &phaseShift);
     } else {
       sendZero(&m, &phaseShift);
@@ -20,23 +75,48 @@ void sendBell202(uint8_t byt) {
   }
 }
 
-void sendBell202buf(uint8_t *buf, size_t len) {
-  m = micros();
+void sendBell202buf(uint8_t *buf, size_t len, bool sync) {
   for (size_t i = 0; i < len; ++i) {
-    sendBell202(buf[i]);
+    sendBell202(buf[i], sync);
+    sync = false;
   }
 }
 
+void initFrame() {
+  memset(txdelay, 0x7e, sizeof(txdelay));
+  memset(&frame, 0, sizeof(frame));
+  frame.sflag = 0x7e;
+  memcpy(&frame.daddr, "GPS\x00\x00\x00\x00", 7);
+  memcpy(&frame.saddr, CALLSIGN, 6);
+  frame.ctrl = 0x03;
+  frame.proto = 0xf0;
+  frame.aprs.dtid = 1;
+  memcpy(&frame.aprs.time, "151430h", 7);
+  memcpy(&frame.aprs.lat, "4730.00N", 8);
+  frame.aprs.sym_table_id = '/';
+  memcpy(&frame.aprs.lon, "00730.00E", 9);
+  frame.aprs.sym_code = BICYCLE;
+  frame.fcs = 0xffff;
+  for (size_t i = 0; i < sizeof(frame) - 3; ++i) {
+    frame.fcs = _crc_xmodem_update(frame.fcs, ((uint8_t*) &frame)[i]);
+  }
+  frame.eflag = 0x7e;
+}
+
 
 void setup() {
   OUT_DDR = 0xff;
-  Serial.begin(115200);
+  setZero();
+  initFrame();
+  nrzi = true;
 }
 
-uint8_t bytes[] = {
-  0xff, 0x55, 0xff, 0x00, 0xff, 0x00
-};
-
 void loop() {
-  sendBell202buf(bytes, 6);
+  sendBell202buf(txdelay, sizeof(txdelay), true);
+  //sendBell202buf((uint8_t*) &frame, sizeof(frame) - 1, false);
+  //sendBell202buf((uint8_t*) &frame, sizeof(frame) - 1, false);
+  sendBell202buf((uint8_t*) &frame, sizeof(frame), false);
+  setZero();
+  nrzi = true;
+  delay(10000);
 }