diff --git a/Config.h b/Config.h
index da79210..ea3a784 100644
--- a/Config.h
+++ b/Config.h
@@ -78,6 +78,9 @@
 // Use the YSF and P25 LEDs for NXDN
 // #define USE_ALTERNATE_NXDN_LEDS
 
+// Use the D-Star and P25 LEDs for M17
+// #define USE_ALTERNATE_M17_LEDS
+
 // Use the D-Star and DMR LEDs for POCSAG
 // #define USE_ALTERNATE_POCSAG_LEDS
 
diff --git a/Globals.h b/Globals.h
index 5cb147a..760ca32 100644
--- a/Globals.h
+++ b/Globals.h
@@ -50,6 +50,7 @@ enum MMDVM_STATE {
   STATE_P25       = 4,
   STATE_NXDN      = 5,
   STATE_POCSAG    = 6,
+  STATE_M17       = 7,
   STATE_FM        = 10,
 
   // Dummy states start at 90
@@ -84,6 +85,8 @@ enum MMDVM_STATE {
 #include "YSFTX.h"
 #include "P25RX.h"
 #include "P25TX.h"
+#include "M17RX.h"
+#include "M17TX.h"
 #include "NXDNRX.h"
 #include "NXDNTX.h"
 #include "POCSAGTX.h"
@@ -123,6 +126,7 @@ extern bool m_ysfEnable;
 extern bool m_p25Enable;
 extern bool m_nxdnEnable;
 extern bool m_pocsagEnable;
+extern bool m_m17Enable;
 extern bool m_fmEnable;
 
 extern bool m_duplex;
@@ -154,6 +158,9 @@ extern CNXDNTX nxdnTX;
 
 extern CPOCSAGTX pocsagTX;
 
+extern CM17RX m17RX;
+extern CM17TX m17TX;
+
 extern CFM fm;
 
 extern CCalDStarRX calDStarRX;
diff --git a/IO.cpp b/IO.cpp
index 5e10feb..2aebcbb 100644
--- a/IO.cpp
+++ b/IO.cpp
@@ -79,6 +79,7 @@ m_dmrTXLevel(128 * 128),
 m_ysfTXLevel(128 * 128),
 m_p25TXLevel(128 * 128),
 m_nxdnTXLevel(128 * 128),
+m_m17TXLevel(128 * 128),
 m_pocsagTXLevel(128 * 128),
 m_fmTXLevel(128 * 128),
 m_rxDCOffset(DC_OFFSET),
@@ -147,6 +148,9 @@ void CIO::selfTest()
 #if !defined(USE_ALTERNATE_NXDN_LEDS)
     setNXDNInt(ledValue);
 #endif
+#if !defined(USE_ALTERNATE_M17_LEDS)
+    setM17Int(ledValue);
+#endif
 #if !defined(USE_ALTERNATE_POCSAG_LEDS)
     setPOCSAGInt(ledValue);
 #endif
@@ -165,6 +169,9 @@ void CIO::selfTest()
 #if !defined(USE_ALTERNATE_NXDN_LEDS)
   setNXDNInt(false);
 #endif
+#if !defined(USE_ALTERNATE_M17_LEDS)
+  setM17Int(false);
+#endif
 #if !defined(USE_ALTERNATE_POCSAG_LEDS)
   setPOCSAGInt(false);
 #endif
@@ -187,6 +194,11 @@ void CIO::selfTest()
   setNXDNInt(true);
 #endif
 
+#if !defined(USE_ALTERNATE_M17_LEDS)
+  delayInt(250);
+  setM17Int(true);
+#endif
+
 #if !defined(USE_ALTERNATE_POCSAG_LEDS)
   delayInt(250);
   setPOCSAGInt(true);
@@ -205,6 +217,11 @@ void CIO::selfTest()
   setPOCSAGInt(false);
 #endif
 
+#if !defined(USE_ALTERNATE_M17_LEDS)
+  delayInt(250);
+  setM17Int(false);
+#endif
+
 #if !defined(USE_ALTERNATE_NXDN_LEDS)
   delayInt(250);
   setNXDNInt(false);
@@ -242,7 +259,7 @@ void CIO::process()
   if (m_started) {
     // Two seconds timeout
     if (m_watchdog >= 48000U) {
-      if (m_modemState == STATE_DSTAR || m_modemState == STATE_DMR || m_modemState == STATE_YSF || m_modemState == STATE_P25 || m_modemState == STATE_NXDN || m_modemState == STATE_POCSAG) {
+      if (m_modemState == STATE_DSTAR || m_modemState == STATE_DMR || m_modemState == STATE_YSF || m_modemState == STATE_P25 || m_modemState == STATE_NXDN || m_modemState == STATE_M17 || m_modemState == STATE_POCSAG) {
         if (m_modemState == STATE_DMR && m_tx)
           dmrTX.setStart(false);
         setMode(STATE_IDLE);
@@ -353,7 +370,7 @@ void CIO::process()
         nxdnRX.samples(NXDNVals, rssi, RX_BLOCK_SIZE);
       }
 
-      if (m_dmrEnable || m_ysfEnable) {
+      if (m_dmrEnable || m_ysfEnable || m_m17Enable) {
         q15_t RRCVals[RX_BLOCK_SIZE];
         ::arm_fir_fast_q15(&m_rrcFilter, samples, RRCVals, RX_BLOCK_SIZE);
 
@@ -366,6 +383,9 @@ void CIO::process()
           else
             dmrDMORX.samples(RRCVals, rssi, RX_BLOCK_SIZE);
         }
+
+        if (m_m17Enable)
+          m17RX.samples(RRCVals, rssi, RX_BLOCK_SIZE);
       }
 
       if (m_fmEnable) {
@@ -434,6 +454,16 @@ void CIO::process()
 
         nxdnRX.samples(NXDNVals, rssi, RX_BLOCK_SIZE);
       }
+    } else if (m_modemState == STATE_M17) {
+      if (m_m17Enable) {
+        q15_t M17Vals[RX_BLOCK_SIZE];
+#if defined(USE_DCBLOCKER)
+        ::arm_fir_fast_q15(&m_rrcFilter, dcSamples, M17Vals, RX_BLOCK_SIZE);
+#else
+        ::arm_fir_fast_q15(&m_rrcFilter, samples, M17Vals, RX_BLOCK_SIZE);
+#endif
+        m17RX.samples(M17Vals, rssi, RX_BLOCK_SIZE);
+      }
     } else if (m_modemState == STATE_FM) {
       bool cos = getCOSInt();
 #if defined(USE_DCBLOCKER)
@@ -483,6 +513,9 @@ void CIO::write(MMDVM_STATE mode, q15_t* samples, uint16_t length, const uint8_t
     case STATE_NXDN:
       txLevel = m_nxdnTXLevel;
       break;
+    case STATE_M17:
+      txLevel = m_m17TXLevel;
+      break;
     case STATE_POCSAG:
       txLevel = m_pocsagTXLevel;
       break;
@@ -540,6 +573,7 @@ void CIO::setMode(MMDVM_STATE state)
     case STATE_YSF:    setYSFInt(false);    break;
     case STATE_P25:    setP25Int(false);    break;
     case STATE_NXDN:   setNXDNInt(false);   break;
+    case STATE_M17:    setM17Int(false);   break;
     case STATE_POCSAG: setPOCSAGInt(false); break;
     case STATE_FM:     setFMInt(false);     break;
   }
@@ -550,6 +584,7 @@ void CIO::setMode(MMDVM_STATE state)
     case STATE_YSF:    setYSFInt(true);    break;
     case STATE_P25:    setP25Int(true);    break;
     case STATE_NXDN:   setNXDNInt(true);   break;
+    case STATE_M17:    setM17Int(true);    break;
     case STATE_POCSAG: setPOCSAGInt(true); break;
     case STATE_FM:     setFMInt(true);     break;
   }
@@ -558,7 +593,7 @@ void CIO::setMode(MMDVM_STATE state)
   m_modemState = state;
 }
 
-void CIO::setParameters(bool rxInvert, bool txInvert, bool pttInvert, uint8_t rxLevel, uint8_t cwIdTXLevel, uint8_t dstarTXLevel, uint8_t dmrTXLevel, uint8_t ysfTXLevel, uint8_t p25TXLevel, uint8_t nxdnTXLevel, uint8_t pocsagTXLevel, uint8_t fmTXLevel, int16_t txDCOffset, int16_t rxDCOffset, bool useCOSAsLockout)
+void CIO::setParameters(bool rxInvert, bool txInvert, bool pttInvert, uint8_t rxLevel, uint8_t cwIdTXLevel, uint8_t dstarTXLevel, uint8_t dmrTXLevel, uint8_t ysfTXLevel, uint8_t p25TXLevel, uint8_t nxdnTXLevel, uint8_t m17TXLevel, uint8_t pocsagTXLevel, uint8_t fmTXLevel, int16_t txDCOffset, int16_t rxDCOffset, bool useCOSAsLockout)
 {
   m_pttInvert = pttInvert;
 
@@ -569,6 +604,7 @@ void CIO::setParameters(bool rxInvert, bool txInvert, bool pttInvert, uint8_t rx
   m_ysfTXLevel    = q15_t(ysfTXLevel * 128);
   m_p25TXLevel    = q15_t(p25TXLevel * 128);
   m_nxdnTXLevel   = q15_t(nxdnTXLevel * 128);
+  m_m17TXLevel    = q15_t(m17TXLevel * 128);
   m_pocsagTXLevel = q15_t(pocsagTXLevel * 128);
   m_fmTXLevel     = q15_t(fmTXLevel * 128);
 
@@ -586,6 +622,7 @@ void CIO::setParameters(bool rxInvert, bool txInvert, bool pttInvert, uint8_t rx
     m_ysfTXLevel    = -m_ysfTXLevel;
     m_p25TXLevel    = -m_p25TXLevel;
     m_nxdnTXLevel   = -m_nxdnTXLevel;
+    m_m17TXLevel    = -m_m17TXLevel;
     m_pocsagTXLevel = -m_pocsagTXLevel;
   }
 }
diff --git a/IO.h b/IO.h
index cd4d3d1..4364a63 100644
--- a/IO.h
+++ b/IO.h
@@ -42,7 +42,7 @@ public:
   
   void interrupt();
 
-  void setParameters(bool rxInvert, bool txInvert, bool pttInvert, uint8_t rxLevel, uint8_t cwIdTXLevel, uint8_t dstarTXLevel, uint8_t dmrTXLevel, uint8_t ysfTXLevel, uint8_t p25TXLevel, uint8_t nxdnTXLevel, uint8_t pocsagTXLevel, uint8_t fmTXLevel, int16_t txDCOffset, int16_t rxDCOffset, bool useCOSAsLockout);
+  void setParameters(bool rxInvert, bool txInvert, bool pttInvert, uint8_t rxLevel, uint8_t cwIdTXLevel, uint8_t dstarTXLevel, uint8_t dmrTXLevel, uint8_t ysfTXLevel, uint8_t p25TXLevel, uint8_t nxdnTXLevel, uint8_t m17TXLevel, uint8_t pocsagTXLevel, uint8_t fmTXLevel, int16_t txDCOffset, int16_t rxDCOffset, bool useCOSAsLockout);
 
   void getOverflow(bool& adcOverflow, bool& dacOverflow);
 
@@ -85,6 +85,7 @@ private:
   q15_t                m_ysfTXLevel;
   q15_t                m_p25TXLevel;
   q15_t                m_nxdnTXLevel;
+  q15_t                m_m17TXLevel;
   q15_t                m_pocsagTXLevel;
   q15_t                m_fmTXLevel;
 
@@ -121,6 +122,7 @@ private:
   void setP25Int(bool on);
   void setNXDNInt(bool on);
   void setPOCSAGInt(bool on);
+  void setM17Int(bool on);
   void setFMInt(bool on);
   
   void delayInt(unsigned int dly);
diff --git a/M17Defines.h b/M17Defines.h
new file mode 100644
index 0000000..47b02c1
--- /dev/null
+++ b/M17Defines.h
@@ -0,0 +1,47 @@
+/*
+ *   Copyright (C) 2016,2017,2018,2020 by Jonathan Naylor G4KLX
+ *
+ *   This program is free software; you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation; either version 2 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *   GNU General Public License for more details.
+ *
+ *   You should have received a copy of the GNU General Public License
+ *   along with this program; if not, write to the Free Software
+ *   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+#if !defined(M17DEFINES_H)
+#define  M17DEFINES_H
+
+const unsigned int M17_RADIO_SYMBOL_LENGTH = 5U;      // At 24 kHz sample rate
+
+const unsigned int M17_FRAME_LENGTH_BITS    = 384U;
+const unsigned int M17_FRAME_LENGTH_BYTES   = M17_FRAME_LENGTH_BITS / 8U;
+const unsigned int M17_FRAME_LENGTH_SYMBOLS = M17_FRAME_LENGTH_BITS / 2U;
+const unsigned int M17_FRAME_LENGTH_SAMPLES = M17_FRAME_LENGTH_SYMBOLS * M17_RADIO_SYMBOL_LENGTH;
+
+const unsigned int M17_SYNC_LENGTH_BITS    = 16U;
+const unsigned int M17_SYNC_LENGTH_SYMBOLS = M17_SYNC_LENGTH_BITS / 2U;
+const unsigned int M17_SYNC_LENGTH_SAMPLES = M17_SYNC_LENGTH_SYMBOLS * M17_RADIO_SYMBOL_LENGTH;
+
+const uint8_t M17_SYNC_BYTES[]      = {0xCDU, 0xF5U};	// XXX
+const uint8_t M17_SYNC_BYTES_LENGTH = 2U;
+
+const uint16_t M17_SYNC_BITS      = 0xCDF5U;			// XXX
+
+// C     D      F      5					// XXX
+// 11 00 11 01  11 11  01 01					// XXX
+// -3 +1 -3 +3  -3 -3  +3 +3					// XXX
+
+const int8_t M17_SYNC_SYMBOLS_VALUES[] = {-3, +1, -3, +3, -3, -3, +3, +3};	// XXX
+
+const uint16_t M17_SYNC_SYMBOLS = 0x014DU;			// XXX
+
+#endif
+
diff --git a/M17RX.cpp b/M17RX.cpp
new file mode 100644
index 0000000..06e9240
--- /dev/null
+++ b/M17RX.cpp
@@ -0,0 +1,404 @@
+/*
+ *   Copyright (C) 2009-2017,2020 by Jonathan Naylor G4KLX
+ *
+ *   This program is free software; you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation; either version 2 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *   GNU General Public License for more details.
+ *
+ *   You should have received a copy of the GNU General Public License
+ *   along with this program; if not, write to the Free Software
+ *   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+#include "Config.h"
+#include "Globals.h"
+#include "M17RX.h"
+#include "Utils.h"
+
+const q15_t SCALING_FACTOR = 18750;      // Q15(0.55)
+
+const uint8_t MAX_SYNC_BIT_START_ERRS = 2U;
+const uint8_t MAX_SYNC_BIT_RUN_ERRS   = 4U;
+
+const uint8_t MAX_SYNC_SYMBOLS_ERRS = 3U;
+
+const uint8_t BIT_MASK_TABLE[] = {0x80U, 0x40U, 0x20U, 0x10U, 0x08U, 0x04U, 0x02U, 0x01U};
+
+#define WRITE_BIT1(p,i,b) p[(i)>>3] = (b) ? (p[(i)>>3] | BIT_MASK_TABLE[(i)&7]) : (p[(i)>>3] & ~BIT_MASK_TABLE[(i)&7])
+
+const uint8_t NOAVEPTR = 99U;
+
+const uint16_t NOENDPTR = 9999U;
+
+const unsigned int MAX_SYNC_FRAMES = 1U + 1U;
+
+CM17RX::CM17RX() :
+m_state(M17RXS_NONE),
+m_bitBuffer(),
+m_buffer(),
+m_bitPtr(0U),
+m_dataPtr(0U),
+m_startPtr(NOENDPTR),
+m_endPtr(NOENDPTR),
+m_syncPtr(NOENDPTR),
+m_minSyncPtr(NOENDPTR),
+m_maxSyncPtr(NOENDPTR),
+m_maxCorr(0),
+m_lostCount(0U),
+m_countdown(0U),
+m_centre(),
+m_centreVal(0),
+m_threshold(),
+m_thresholdVal(0),
+m_averagePtr(NOAVEPTR),
+m_rssiAccum(0U),
+m_rssiCount(0U)
+{
+}
+
+void CM17RX::reset()
+{
+  m_state        = M17RXS_NONE;
+  m_dataPtr      = 0U;
+  m_bitPtr       = 0U;
+  m_maxCorr      = 0;
+  m_averagePtr   = NOAVEPTR;
+  m_startPtr     = NOENDPTR;
+  m_endPtr       = NOENDPTR;
+  m_syncPtr      = NOENDPTR;
+  m_minSyncPtr   = NOENDPTR;
+  m_maxSyncPtr   = NOENDPTR;
+  m_centreVal    = 0;
+  m_thresholdVal = 0;
+  m_lostCount    = 0U;
+  m_countdown    = 0U;
+  m_rssiAccum    = 0U;
+  m_rssiCount    = 0U;
+}
+
+void CM17RX::samples(const q15_t* samples, uint16_t* rssi, uint8_t length)
+{
+  for (uint8_t i = 0U; i < length; i++) {
+    q15_t sample = samples[i];
+
+    m_rssiAccum += rssi[i];
+    m_rssiCount++;
+
+    m_bitBuffer[m_bitPtr] <<= 1;
+    if (sample < 0)
+      m_bitBuffer[m_bitPtr] |= 0x01U;
+
+    m_buffer[m_dataPtr] = sample;
+
+    switch (m_state) {
+    case M17RXS_DATA:
+      processData(sample);
+      break;
+    default:
+      processNone(sample);
+      break;
+    }
+
+    m_dataPtr++;
+    if (m_dataPtr >= M17_FRAME_LENGTH_SAMPLES)
+      m_dataPtr = 0U;
+
+    m_bitPtr++;
+    if (m_bitPtr >= M17_RADIO_SYMBOL_LENGTH)
+      m_bitPtr = 0U;
+  }
+}
+
+void CM17RX::processNone(q15_t sample)
+{
+  bool ret = correlateSync();
+  if (ret) {
+    // On the first sync, start the countdown to the state change
+    if (m_countdown == 0U) {
+      m_rssiAccum = 0U;
+      m_rssiCount = 0U;
+
+      io.setDecode(true);
+      io.setADCDetection(true);
+
+      m_averagePtr = NOAVEPTR;
+
+      m_countdown = 5U;
+    }
+  }
+
+  if (m_countdown > 0U)
+    m_countdown--;
+
+  if (m_countdown == 1U) {
+    m_minSyncPtr = m_syncPtr + M17_FRAME_LENGTH_SAMPLES - 1U;
+    if (m_minSyncPtr >= M17_FRAME_LENGTH_SAMPLES)
+      m_minSyncPtr -= M17_FRAME_LENGTH_SAMPLES;
+
+    m_maxSyncPtr = m_syncPtr + 1U;
+    if (m_maxSyncPtr >= M17_FRAME_LENGTH_SAMPLES)
+      m_maxSyncPtr -= M17_FRAME_LENGTH_SAMPLES;
+
+    m_state      = M17RXS_DATA;
+    m_countdown  = 0U;
+  }
+}
+
+void CM17RX::processData(q15_t sample)
+{
+  if (m_minSyncPtr < m_maxSyncPtr) {
+    if (m_dataPtr >= m_minSyncPtr && m_dataPtr <= m_maxSyncPtr)
+      correlateSync();
+  } else {
+    if (m_dataPtr >= m_minSyncPtr || m_dataPtr <= m_maxSyncPtr)
+      correlateSync();
+  }
+
+  if (m_dataPtr == m_endPtr) {
+    // Only update the centre and threshold if they are from a good sync
+    if (m_lostCount == MAX_SYNC_FRAMES) {
+      m_minSyncPtr = m_syncPtr + M17_FRAME_LENGTH_SAMPLES - 1U;
+      if (m_minSyncPtr >= M17_FRAME_LENGTH_SAMPLES)
+        m_minSyncPtr -= M17_FRAME_LENGTH_SAMPLES;
+
+      m_maxSyncPtr = m_syncPtr + 1U;
+      if (m_maxSyncPtr >= M17_FRAME_LENGTH_SAMPLES)
+        m_maxSyncPtr -= M17_FRAME_LENGTH_SAMPLES;
+    }
+
+    calculateLevels(m_startPtr, M17_FRAME_LENGTH_SYMBOLS);
+
+    DEBUG4("M17RX: sync found pos/centre/threshold", m_syncPtr, m_centreVal, m_thresholdVal);
+
+    uint8_t frame[M17_FRAME_LENGTH_BYTES + 3U];
+    samplesToBits(m_startPtr, M17_FRAME_LENGTH_SYMBOLS, frame, 8U, m_centreVal, m_thresholdVal);
+
+    // We've not seen a data sync for too long, signal RXLOST and change to RX_NONE
+    m_lostCount--;
+    if (m_lostCount == 0U) {
+      DEBUG1("M17RX: sync timed out, lost lock");
+
+      io.setDecode(false);
+      io.setADCDetection(false);
+
+      serial.writeM17Lost();
+
+      m_state      = M17RXS_NONE;
+      m_endPtr     = NOENDPTR;
+      m_averagePtr = NOAVEPTR;
+      m_countdown  = 0U;
+      m_maxCorr    = 0;
+    } else {
+      frame[0U] = m_lostCount == (MAX_SYNC_FRAMES - 1U) ? 0x01U : 0x00U;
+      writeRSSIData(frame);
+      m_maxCorr = 0;
+    }
+  }
+}
+
+bool CM17RX::correlateSync()
+{
+  if (countBits16(m_bitBuffer[m_bitPtr] ^ M17_SYNC_SYMBOLS) <= MAX_SYNC_SYMBOLS_ERRS) {
+    uint16_t ptr = m_dataPtr + M17_FRAME_LENGTH_SAMPLES - M17_SYNC_LENGTH_SAMPLES + M17_RADIO_SYMBOL_LENGTH;
+    if (ptr >= M17_FRAME_LENGTH_SAMPLES)
+      ptr -= M17_FRAME_LENGTH_SAMPLES;
+
+    q31_t corr = 0;
+    q15_t min =  16000;
+    q15_t max = -16000;
+
+    for (uint8_t i = 0U; i < M17_SYNC_LENGTH_SYMBOLS; i++) {
+      q15_t val = m_buffer[ptr];
+
+      if (val > max)
+        max = val;
+      if (val < min)
+        min = val;
+
+      switch (M17_SYNC_SYMBOLS_VALUES[i]) {
+      case +3:
+        corr -= (val + val + val);
+        break;
+      case +1:
+        corr -= val;
+        break;
+      case -1:
+        corr += val;
+        break;
+      default:  // -3
+        corr += (val + val + val);
+        break;
+      }
+
+      ptr += M17_RADIO_SYMBOL_LENGTH;
+      if (ptr >= M17_FRAME_LENGTH_SAMPLES)
+        ptr -= M17_FRAME_LENGTH_SAMPLES;
+    }
+
+    if (corr > m_maxCorr) {
+      if (m_averagePtr == NOAVEPTR) {
+        m_centreVal = (max + min) >> 1;
+
+        q31_t v1 = (max - m_centreVal) * SCALING_FACTOR;
+        m_thresholdVal = q15_t(v1 >> 15);
+      }
+
+      uint16_t startPtr = m_dataPtr + M17_FRAME_LENGTH_SAMPLES - M17_SYNC_LENGTH_SAMPLES + M17_RADIO_SYMBOL_LENGTH;
+      if (startPtr >= M17_FRAME_LENGTH_SAMPLES)
+        startPtr -= M17_FRAME_LENGTH_SAMPLES;
+
+      uint8_t sync[M17_SYNC_BYTES_LENGTH];
+      samplesToBits(startPtr, M17_SYNC_LENGTH_SYMBOLS, sync, 0U, m_centreVal, m_thresholdVal);
+
+      uint8_t maxErrs;
+      if (m_state == M17RXS_NONE)
+        maxErrs = MAX_SYNC_BIT_START_ERRS;
+      else
+        maxErrs = MAX_SYNC_BIT_RUN_ERRS;
+
+      uint8_t errs = 0U;
+      for (uint8_t i = 0U; i < M17_SYNC_BYTES_LENGTH; i++)
+        errs += countBits8(sync[i] ^ M17_SYNC_BYTES[i]);
+
+      if (errs <= maxErrs) {
+        m_maxCorr   = corr;
+        m_lostCount = MAX_SYNC_FRAMES;
+        m_syncPtr   = m_dataPtr;
+
+        m_startPtr = startPtr;
+
+        m_endPtr = m_dataPtr + M17_FRAME_LENGTH_SAMPLES - M17_SYNC_LENGTH_SAMPLES - 1U;
+        if (m_endPtr >= M17_FRAME_LENGTH_SAMPLES)
+          m_endPtr -= M17_FRAME_LENGTH_SAMPLES;
+
+        return true;
+      }
+    }
+  }
+
+  return false;
+}
+
+void CM17RX::calculateLevels(uint16_t start, uint16_t count)
+{
+  q15_t maxPos = -16000;
+  q15_t minPos =  16000;
+  q15_t maxNeg =  16000;
+  q15_t minNeg = -16000;
+
+  for (uint16_t i = 0U; i < count; i++) {
+    q15_t sample = m_buffer[start];
+
+    if (sample > 0) {
+      if (sample > maxPos)
+        maxPos = sample;
+      if (sample < minPos)
+        minPos = sample;
+    } else {
+      if (sample < maxNeg)
+        maxNeg = sample;
+      if (sample > minNeg)
+        minNeg = sample;
+    }
+
+    start += M17_RADIO_SYMBOL_LENGTH;
+    if (start >= M17_FRAME_LENGTH_SAMPLES)
+      start -= M17_FRAME_LENGTH_SAMPLES;
+  }
+
+  q15_t posThresh = (maxPos + minPos) >> 1;
+  q15_t negThresh = (maxNeg + minNeg) >> 1;
+
+  q15_t centre = (posThresh + negThresh) >> 1;
+
+  q15_t threshold = posThresh - centre;
+
+  DEBUG5("M17RX: pos/neg/centre/threshold", posThresh, negThresh, centre, threshold);
+
+  if (m_averagePtr == NOAVEPTR) {
+    for (uint8_t i = 0U; i < 16U; i++) {
+      m_centre[i] = centre;
+      m_threshold[i] = threshold;
+    }
+
+    m_averagePtr = 0U;
+  } else {
+    m_centre[m_averagePtr] = centre;
+    m_threshold[m_averagePtr] = threshold;
+
+    m_averagePtr++;
+    if (m_averagePtr >= 16U)
+      m_averagePtr = 0U;
+  }
+
+  m_centreVal = 0;
+  m_thresholdVal = 0;
+
+  for (uint8_t i = 0U; i < 16U; i++) {
+    m_centreVal += m_centre[i];
+    m_thresholdVal += m_threshold[i];
+  }
+
+  m_centreVal >>= 4;
+  m_thresholdVal >>= 4;
+}
+
+void CM17RX::samplesToBits(uint16_t start, uint16_t count, uint8_t* buffer, uint16_t offset, q15_t centre, q15_t threshold)
+{
+  for (uint16_t i = 0U; i < count; i++) {
+    q15_t sample = m_buffer[start] - centre;
+
+    if (sample < -threshold) {
+      WRITE_BIT1(buffer, offset, false);
+      offset++;
+      WRITE_BIT1(buffer, offset, true);
+      offset++;
+    } else if (sample < 0) {
+      WRITE_BIT1(buffer, offset, false);
+      offset++;
+      WRITE_BIT1(buffer, offset, false);
+      offset++;
+    } else if (sample < threshold) {
+      WRITE_BIT1(buffer, offset, true);
+      offset++;
+      WRITE_BIT1(buffer, offset, false);
+      offset++;
+    } else {
+      WRITE_BIT1(buffer, offset, true);
+      offset++;
+      WRITE_BIT1(buffer, offset, true);
+      offset++;
+    }
+
+    start += M17_RADIO_SYMBOL_LENGTH;
+    if (start >= M17_FRAME_LENGTH_SAMPLES)
+      start -= M17_FRAME_LENGTH_SAMPLES;
+  }
+}
+
+void CM17RX::writeRSSIData(uint8_t* data)
+{
+#if defined(SEND_RSSI_DATA)
+  if (m_rssiCount > 0U) {
+    uint16_t rssi = m_rssiAccum / m_rssiCount;
+
+    data[121U] = (rssi >> 8) & 0xFFU;
+    data[122U] = (rssi >> 0) & 0xFFU;
+
+    serial.writeM17Data(data, M17_FRAME_LENGTH_BYTES + 3U);
+  } else {
+    serial.writeM17Data(data, M17_FRAME_LENGTH_BYTES + 1U);
+  }
+#else
+  serial.writeM17Data(data, M17_FRAME_LENGTH_BYTES + 1U);
+#endif
+
+  m_rssiAccum = 0U;
+  m_rssiCount = 0U;
+}
diff --git a/M17RX.h b/M17RX.h
new file mode 100644
index 0000000..7d6dc6f
--- /dev/null
+++ b/M17RX.h
@@ -0,0 +1,69 @@
+/*
+ *   Copyright (C) 2015,2016,2017,2020 by Jonathan Naylor G4KLX
+ *
+ *   This program is free software; you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation; either version 2 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *   GNU General Public License for more details.
+ *
+ *   You should have received a copy of the GNU General Public License
+ *   along with this program; if not, write to the Free Software
+ *   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+#if !defined(M17RX_H)
+#define  M17RX_H
+
+#include "Config.h"
+#include "M17Defines.h"
+
+enum M17RX_STATE {
+  M17RXS_NONE,
+  M17RXS_DATA
+};
+
+class CM17RX {
+public:
+  CM17RX();
+
+  void samples(const q15_t* samples, uint16_t* rssi, uint8_t length);
+
+  void reset();
+
+private:
+  M17RX_STATE m_state;
+  uint16_t    m_bitBuffer[M17_RADIO_SYMBOL_LENGTH];
+  q15_t       m_buffer[M17_FRAME_LENGTH_SAMPLES];
+  uint16_t    m_bitPtr;
+  uint16_t    m_dataPtr;
+  uint16_t    m_startPtr;
+  uint16_t    m_endPtr;
+  uint16_t    m_syncPtr;
+  uint16_t    m_minSyncPtr;
+  uint16_t    m_maxSyncPtr;
+  q31_t       m_maxCorr;
+  uint16_t    m_lostCount;
+  uint8_t     m_countdown;
+  q15_t       m_centre[16U];
+  q15_t       m_centreVal;
+  q15_t       m_threshold[16U];
+  q15_t       m_thresholdVal;
+  uint8_t     m_averagePtr;
+  uint32_t    m_rssiAccum;
+  uint16_t    m_rssiCount;
+
+  void processNone(q15_t sample);
+  void processData(q15_t sample);
+  bool correlateSync();
+  void calculateLevels(uint16_t start, uint16_t count);
+  void samplesToBits(uint16_t start, uint16_t count, uint8_t* buffer, uint16_t offset, q15_t centre, q15_t threshold);
+  void writeRSSIData(uint8_t* data);
+};
+
+#endif
+
diff --git a/M17TX.cpp b/M17TX.cpp
new file mode 100644
index 0000000..e38a4db
--- /dev/null
+++ b/M17TX.cpp
@@ -0,0 +1,183 @@
+/*
+ *   Copyright (C) 2009-2018,2020 by Jonathan Naylor G4KLX
+ *   Copyright (C) 2017 by Andy Uribe CA6JAU
+ *
+ *   This program is free software; you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation; either version 2 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *   GNU General Public License for more details.
+ *
+ *   You should have received a copy of the GNU General Public License
+ *   along with this program; if not, write to the Free Software
+ *   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+#include "Config.h"
+#include "Globals.h"
+#include "M17TX.h"
+
+#include "M17Defines.h"
+
+// Generated using rcosdesign(0.2, 8, 5, 'sqrt') in MATLAB
+static q15_t RRC_0_2_FILTER[] = {0, 0, 0, 0, 850, 219, -720, -1548, -1795, -1172, 237, 1927, 3120, 3073, 1447, -1431, -4544, -6442,
+                                 -5735, -1633, 5651, 14822, 23810, 30367, 32767, 30367, 23810, 14822, 5651, -1633, -5735, -6442,
+                                 -4544, -1431, 1447, 3073, 3120, 1927, 237, -1172, -1795, -1548, -720, 219, 850}; // numTaps = 45, L = 5
+const uint16_t RRC_0_2_FILTER_PHASE_LEN = 9U; // phaseLength = numTaps/L
+
+const q15_t M17_LEVELA =  948;
+const q15_t M17_LEVELB =  316;
+const q15_t M17_LEVELC = -316;
+const q15_t M17_LEVELD = -948;
+
+const uint8_t M17_START_SYNC = 0x77U;
+const uint8_t M17_END_SYNC   = 0xFFU;
+const uint8_t M17_HANG       = 0x00U;
+
+CM17TX::CM17TX() :
+m_buffer(TX_BUFFER_LEN),
+m_modFilter(),
+m_modState(),
+m_poBuffer(),
+m_poLen(0U),
+m_poPtr(0U),
+m_txDelay(240U),      // 200ms
+m_txHang(4800U),      // 4s
+m_txCount(0U)
+{
+  ::memset(m_modState, 0x00U, 16U * sizeof(q15_t));
+
+  m_modFilter.L           = M17_RADIO_SYMBOL_LENGTH;
+  m_modFilter.phaseLength = RRC_0_2_FILTER_PHASE_LEN;
+  m_modFilter.pCoeffs     = RRC_0_2_FILTER;
+  m_modFilter.pState      = m_modState;
+}
+
+
+void CM17TX::process()
+{
+  // If we have M17 data to transmit, do so.
+  if (m_poLen == 0U && m_buffer.getData() > 0U) {
+    if (!m_tx) {
+      for (uint16_t i = 0U; i < m_txDelay; i++)
+        m_poBuffer[m_poLen++] = M17_START_SYNC;
+    } else {
+      for (uint8_t i = 0U; i < M17_FRAME_LENGTH_BYTES; i++) {
+        uint8_t c = m_buffer.get();
+        m_poBuffer[m_poLen++] = c;
+      }
+    }
+
+    m_poPtr = 0U;
+  }
+
+  if (m_poLen > 0U) {
+    // Transmit M17 data.
+    uint16_t space = io.getSpace();
+
+    while (space > (4U * M17_RADIO_SYMBOL_LENGTH)) {
+      uint8_t c = m_poBuffer[m_poPtr++];
+      writeByte(c);
+
+      // Reduce space and reset the hang timer.
+      space -= 4U * M17_RADIO_SYMBOL_LENGTH;
+      if (m_duplex)
+        m_txCount = m_txHang;
+
+      if (m_poPtr >= m_poLen) {
+        m_poPtr = 0U;
+        m_poLen = 0U;
+        return;
+      }
+    }
+  } else if (m_txCount > 0U) {
+    // Transmit silence until the hang timer has expired.
+    uint16_t space = io.getSpace();
+
+    while (space > (4U * M17_RADIO_SYMBOL_LENGTH)) {
+      writeSilence();
+
+      space -= 4U * M17_RADIO_SYMBOL_LENGTH;
+      m_txCount--;
+
+      if (m_txCount == 0U)
+        return;
+    }
+  }
+}
+
+uint8_t CM17TX::writeData(const uint8_t* data, uint8_t length)
+{
+  if (length != (M17_FRAME_LENGTH_BYTES + 1U))
+    return 4U;
+
+  uint16_t space = m_buffer.getSpace();
+  if (space < M17_FRAME_LENGTH_BYTES)
+    return 5U;
+
+  for (uint8_t i = 0U; i < M17_FRAME_LENGTH_BYTES; i++)
+    m_buffer.put(data[i + 1U]);
+
+  return 0U;
+}
+
+void CM17TX::writeByte(uint8_t c)
+{
+  q15_t inBuffer[4U];
+  q15_t outBuffer[M17_RADIO_SYMBOL_LENGTH * 4U];
+
+  const uint8_t MASK = 0xC0U;
+
+  for (uint8_t i = 0U; i < 4U; i++, c <<= 2) {
+    switch (c & MASK) {
+      case 0xC0U:
+        inBuffer[i] = M17_LEVELA;
+        break;
+      case 0x80U:
+        inBuffer[i] = M17_LEVELB;
+        break;
+      case 0x00U:
+        inBuffer[i] = M17_LEVELC;
+        break;
+      default:
+        inBuffer[i] = M17_LEVELD;
+        break;
+    }
+  }
+
+  ::arm_fir_interpolate_q15(&m_modFilter, inBuffer, outBuffer, 4U);
+
+  io.write(STATE_M17, outBuffer, M17_RADIO_SYMBOL_LENGTH * 4U);
+}
+
+void CM17TX::writeSilence()
+{
+  q15_t inBuffer[4U] = {0x00U, 0x00U, 0x00U, 0x00U};
+  q15_t outBuffer[M17_RADIO_SYMBOL_LENGTH * 4U];
+
+  ::arm_fir_interpolate_q15(&m_modFilter, inBuffer, outBuffer, 4U);
+
+  io.write(STATE_M17, outBuffer, M17_RADIO_SYMBOL_LENGTH * 4U);
+}
+
+void CM17TX::setTXDelay(uint8_t delay)
+{
+  m_txDelay = 600U + uint16_t(delay) * 12U;        // 500ms + tx delay
+
+  if (m_txDelay > 1200U)
+    m_txDelay = 1200U;
+}
+
+uint8_t CM17TX::getSpace() const
+{
+  return m_buffer.getSpace() / M17_FRAME_LENGTH_BYTES;
+}
+
+void CM17TX::setParams(uint8_t txHang)
+{
+  m_txHang = txHang * 1200U;
+}
diff --git a/M17TX.h b/M17TX.h
new file mode 100644
index 0000000..211d747
--- /dev/null
+++ b/M17TX.h
@@ -0,0 +1,56 @@
+/*
+ *   Copyright (C) 2015,2016,2017,2020 by Jonathan Naylor G4KLX
+ *
+ *   This program is free software; you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation; either version 2 of the License, or
+ *   (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *   GNU General Public License for more details.
+ *
+ *   You should have received a copy of the GNU General Public License
+ *   along with this program; if not, write to the Free Software
+ *   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+#if !defined(M17TX_H)
+#define  M17TX_H
+
+#include "Config.h"
+
+#include "SerialRB.h"
+
+class CM17TX {
+public:
+  CM17TX();
+
+  uint8_t writeData(const uint8_t* data, uint8_t length);
+
+  void process();
+
+  void setTXDelay(uint8_t delay);
+
+  uint8_t getSpace() const;
+
+  void setParams(uint8_t txHang);
+
+private:
+  CSerialRB                        m_buffer;
+  arm_fir_interpolate_instance_q15 m_modFilter;
+  q15_t                            m_modState[16U];    // blockSize + phaseLength - 1, 4 + 9 - 1 plus some spare
+  uint8_t                          m_poBuffer[1200U];
+  uint16_t                         m_poLen;
+  uint16_t                         m_poPtr;
+  uint16_t                         m_txDelay;
+  uint32_t                         m_txHang;
+  uint32_t                         m_txCount;
+
+  void writeByte(uint8_t c);
+  void writeSilence();
+};
+
+#endif
+
diff --git a/MMDVM.cpp b/MMDVM.cpp
index a6d4577..5fadb0a 100644
--- a/MMDVM.cpp
+++ b/MMDVM.cpp
@@ -31,6 +31,7 @@ bool m_dmrEnable    = true;
 bool m_ysfEnable    = true;
 bool m_p25Enable    = true;
 bool m_nxdnEnable   = true;
+bool m_m17Enable    = true;
 bool m_pocsagEnable = true;
 bool m_fmEnable     = true;
 
@@ -58,6 +59,9 @@ CP25TX     p25TX;
 CNXDNRX    nxdnRX;
 CNXDNTX    nxdnTX;
 
+CM17RX     m17RX;
+CM17TX     m17TX;
+
 CPOCSAGTX  pocsagTX;
 
 CFM        fm;
@@ -107,6 +111,9 @@ void loop()
   if (m_nxdnEnable && m_modemState == STATE_NXDN)
     nxdnTX.process();
 
+  if (m_m17Enable && m_modemState == STATE_M17)
+    m17TX.process();
+
   if (m_pocsagEnable && (m_modemState == STATE_POCSAG || pocsagTX.busy()))
     pocsagTX.process();
 
diff --git a/MMDVM.ino b/MMDVM.ino
index 17c4ad8..02bfe62 100644
--- a/MMDVM.ino
+++ b/MMDVM.ino
@@ -28,6 +28,7 @@ bool m_dmrEnable    = true;
 bool m_ysfEnable    = true;
 bool m_p25Enable    = true;
 bool m_nxdnEnable   = true;
+bool m_m17Enable    = true;
 bool m_pocsagEnable = true;
 bool m_fmEnable     = true;
 
@@ -55,6 +56,9 @@ CP25TX     p25TX;
 CNXDNRX    nxdnRX;
 CNXDNTX    nxdnTX;
 
+CM17RX     m17RX;
+CM17TX     m17TX;
+
 CPOCSAGTX  pocsagTX;
 
 CFM        fm;
@@ -104,6 +108,9 @@ void loop()
   if (m_nxdnEnable && m_modemState == STATE_NXDN)
     nxdnTX.process();
 
+  if (m_m17Enable && m_modemState == STATE_M17)
+    m17TX.process();
+
   if (m_pocsagEnable && (m_modemState == STATE_POCSAG || pocsagTX.busy()))
     pocsagTX.process();
 
diff --git a/SerialPort.cpp b/SerialPort.cpp
index 17cb476..20c2766 100644
--- a/SerialPort.cpp
+++ b/SerialPort.cpp
@@ -63,6 +63,9 @@ const uint8_t MMDVM_P25_LOST     = 0x32U;
 const uint8_t MMDVM_NXDN_DATA    = 0x40U;
 const uint8_t MMDVM_NXDN_LOST    = 0x41U;
 
+const uint8_t MMDVM_M17_DATA     = 0x45U;
+const uint8_t MMDVM_M17_LOST     = 0x46U;
+
 const uint8_t MMDVM_POCSAG_DATA  = 0x50U;
 
 const uint8_t MMDVM_FM_PARAMS1   = 0x60U;
@@ -103,7 +106,7 @@ const uint8_t MMDVM_DEBUG5       = 0xF5U;
 #define	HW_TYPE	"MMDVM"
 #endif
 
-#define DESCRIPTION "20200901 (D-Star/DMR/System Fusion/P25/NXDN/POCSAG/FM)"
+#define DESCRIPTION "20201014 (D-Star/DMR/System Fusion/P25/NXDN/M17/POCSAG/FM)"
 
 #if defined(GITVERSION)
 #define concat(h, a, b, c) h " " a " " b " GitID #" c ""
@@ -158,7 +161,7 @@ void CSerialPort::getStatus()
 
   // Send all sorts of interesting internal values
   reply[0U]  = MMDVM_FRAME_START;
-  reply[1U]  = 13U;
+  reply[1U]  = 14U;
   reply[2U]  = MMDVM_GET_STATUS;
 
   reply[3U]  = 0x00U;
@@ -176,6 +179,8 @@ void CSerialPort::getStatus()
     reply[3U] |= 0x20U;
   if (m_fmEnable)
     reply[3U] |= 0x40U;
+  if (m_m17Enable)
+    reply[3U] |= 0x80U;
 
   reply[4U]  = uint8_t(m_modemState);
 
@@ -240,7 +245,12 @@ void CSerialPort::getStatus()
   else
     reply[12U] = 0U;
 
-  writeInt(1U, reply, 13);
+  if (m_m17Enable)
+    reply[13U] = m17TX.getSpace();
+  else
+    reply[13U] = 0U;
+
+  writeInt(1U, reply, 14);
 }
 
 void CSerialPort::getVersion()
@@ -264,7 +274,7 @@ void CSerialPort::getVersion()
 
 uint8_t CSerialPort::setConfig(const uint8_t* data, uint8_t length)
 {
-  if (length < 21U)
+  if (length < 23U)
     return 4U;
 
   bool rxInvert        = (data[0U] & 0x01U) == 0x01U;
@@ -283,6 +293,7 @@ uint8_t CSerialPort::setConfig(const uint8_t* data, uint8_t length)
   bool nxdnEnable   = (data[1U] & 0x10U) == 0x10U;
   bool pocsagEnable = (data[1U] & 0x20U) == 0x20U;
   bool fmEnable     = (data[1U] & 0x40U) == 0x40U;
+  bool m17Enable    = (data[1U] & 0x80U) == 0x80U;
 
   uint8_t txDelay = data[2U];
   if (txDelay > 50U)
@@ -290,7 +301,7 @@ uint8_t CSerialPort::setConfig(const uint8_t* data, uint8_t length)
 
   MMDVM_STATE modemState = MMDVM_STATE(data[3U]);
 
-  if (modemState != STATE_IDLE && modemState != STATE_DSTAR && modemState != STATE_DMR && modemState != STATE_YSF && modemState != STATE_P25 && modemState != STATE_NXDN && modemState != STATE_POCSAG && modemState != STATE_FM && modemState != STATE_DSTARCAL && modemState != STATE_DMRCAL && modemState != STATE_RSSICAL && modemState != STATE_LFCAL && modemState != STATE_DMRCAL1K && modemState != STATE_P25CAL1K && modemState != STATE_DMRDMO1K && modemState != STATE_NXDNCAL1K && modemState != STATE_POCSAGCAL && modemState != STATE_FMCAL10K && modemState != STATE_FMCAL12K && modemState != STATE_FMCAL15K && modemState != STATE_FMCAL20K && modemState != STATE_FMCAL25K && modemState != STATE_FMCAL30K)
+  if (modemState != STATE_IDLE && modemState != STATE_DSTAR && modemState != STATE_DMR && modemState != STATE_YSF && modemState != STATE_P25 && modemState != STATE_NXDN && modemState != STATE_M17 && modemState != STATE_POCSAG && modemState != STATE_FM && modemState != STATE_DSTARCAL && modemState != STATE_DMRCAL && modemState != STATE_RSSICAL && modemState != STATE_LFCAL && modemState != STATE_DMRCAL1K && modemState != STATE_P25CAL1K && modemState != STATE_DMRDMO1K && modemState != STATE_NXDNCAL1K && modemState != STATE_POCSAGCAL && modemState != STATE_FMCAL10K && modemState != STATE_FMCAL12K && modemState != STATE_FMCAL15K && modemState != STATE_FMCAL20K && modemState != STATE_FMCAL25K && modemState != STATE_FMCAL30K)
     return 4U;
   if (modemState == STATE_DSTAR && !dstarEnable)
     return 4U;
@@ -302,6 +313,8 @@ uint8_t CSerialPort::setConfig(const uint8_t* data, uint8_t length)
     return 4U;
   if (modemState == STATE_NXDN && !nxdnEnable)
     return 4U;
+  if (modemState == STATE_M17 && !m17Enable)
+    return 4U;
   if (modemState == STATE_POCSAG && !pocsagEnable)
     return 4U;
   if (modemState == STATE_FM && !fmEnable)
@@ -336,6 +349,9 @@ uint8_t CSerialPort::setConfig(const uint8_t* data, uint8_t length)
 
   uint8_t nxdnTXHang    = data[20U];
 
+  uint8_t m17TXLevel    = data[21U];
+  uint8_t m17TXHang     = data[22U];
+
   setMode(modemState);
 
   m_dstarEnable  = dstarEnable;
@@ -343,6 +359,7 @@ uint8_t CSerialPort::setConfig(const uint8_t* data, uint8_t length)
   m_ysfEnable    = ysfEnable;
   m_p25Enable    = p25Enable;
   m_nxdnEnable   = nxdnEnable;
+  m_m17Enable    = m17Enable;
   m_pocsagEnable = pocsagEnable;
   m_fmEnable     = fmEnable;
   m_duplex       = !simplex;
@@ -352,6 +369,7 @@ uint8_t CSerialPort::setConfig(const uint8_t* data, uint8_t length)
   p25TX.setTXDelay(txDelay);
   dmrDMOTX.setTXDelay(txDelay);
   nxdnTX.setTXDelay(txDelay);
+  m17TX.setTXDelay(txDelay);
   pocsagTX.setTXDelay(txDelay);
 
   dmrTX.setColorCode(colorCode);
@@ -363,8 +381,9 @@ uint8_t CSerialPort::setConfig(const uint8_t* data, uint8_t length)
   ysfTX.setParams(ysfLoDev, ysfTXHang);
   p25TX.setParams(p25TXHang);
   nxdnTX.setParams(nxdnTXHang);
+  m17TX.setParams(m17TXHang);
 
-  io.setParameters(rxInvert, txInvert, pttInvert, rxLevel, cwIdTXLevel, dstarTXLevel, dmrTXLevel, ysfTXLevel, p25TXLevel, nxdnTXLevel, pocsagTXLevel, fmTXLevel, txDCOffset, rxDCOffset, useCOSAsLockout);
+  io.setParameters(rxInvert, txInvert, pttInvert, rxLevel, cwIdTXLevel, dstarTXLevel, dmrTXLevel, ysfTXLevel, p25TXLevel, nxdnTXLevel, m17TXLevel, pocsagTXLevel, fmTXLevel, txDCOffset, rxDCOffset, useCOSAsLockout);
 
   io.start();
 
@@ -452,7 +471,7 @@ uint8_t CSerialPort::setMode(const uint8_t* data, uint8_t length)
   if (modemState == m_modemState)
     return 0U;
 
-  if (modemState != STATE_IDLE && modemState != STATE_DSTAR && modemState != STATE_DMR && modemState != STATE_YSF && modemState != STATE_P25 && modemState != STATE_NXDN && modemState != STATE_POCSAG && modemState != STATE_FM && modemState != STATE_DSTARCAL && modemState != STATE_DMRCAL && modemState != STATE_RSSICAL && modemState != STATE_LFCAL && modemState != STATE_DMRCAL1K && modemState != STATE_P25CAL1K && modemState != STATE_DMRDMO1K && modemState != STATE_NXDNCAL1K && modemState != STATE_POCSAGCAL && modemState != STATE_FMCAL10K  && modemState != STATE_FMCAL12K && modemState != STATE_FMCAL15K && modemState != STATE_FMCAL20K && modemState != STATE_FMCAL25K && modemState != STATE_FMCAL30K)
+  if (modemState != STATE_IDLE && modemState != STATE_DSTAR && modemState != STATE_DMR && modemState != STATE_YSF && modemState != STATE_P25 && modemState != STATE_NXDN && modemState != STATE_M17 && modemState != STATE_POCSAG && modemState != STATE_FM && modemState != STATE_DSTARCAL && modemState != STATE_DMRCAL && modemState != STATE_RSSICAL && modemState != STATE_LFCAL && modemState != STATE_DMRCAL1K && modemState != STATE_P25CAL1K && modemState != STATE_DMRDMO1K && modemState != STATE_NXDNCAL1K && modemState != STATE_POCSAGCAL && modemState != STATE_FMCAL10K  && modemState != STATE_FMCAL12K && modemState != STATE_FMCAL15K && modemState != STATE_FMCAL20K && modemState != STATE_FMCAL25K && modemState != STATE_FMCAL30K)
     return 4U;
   if (modemState == STATE_DSTAR && !m_dstarEnable)
     return 4U;
@@ -464,6 +483,8 @@ uint8_t CSerialPort::setMode(const uint8_t* data, uint8_t length)
     return 4U;
   if (modemState == STATE_NXDN && !m_nxdnEnable)
     return 4U;
+  if (modemState == STATE_M17 && !m_m17Enable)
+    return 4U;
   if (modemState == STATE_POCSAG && !m_pocsagEnable)
     return 4U;
   if (modemState == STATE_FM && !m_fmEnable)
@@ -492,6 +513,9 @@ void CSerialPort::setMode(MMDVM_STATE modemState)
     case STATE_NXDN:
       DEBUG1("Mode set to NXDN");
       break;
+    case STATE_M17:
+      DEBUG1("Mode set to M17");
+      break;
     case STATE_POCSAG:
       DEBUG1("Mode set to POCSAG");
       break;
@@ -563,6 +587,9 @@ void CSerialPort::setMode(MMDVM_STATE modemState)
   if (modemState != STATE_NXDN)
     nxdnRX.reset();
 
+  if (modemState != STATE_M17)
+    m17RX.reset();
+
   if (modemState != STATE_FM)
     fm.reset();
 
@@ -870,6 +897,20 @@ void CSerialPort::process()
             }
             break;
 
+          case MMDVM_M17_DATA:
+            if (m_m17Enable) {
+              if (m_modemState == STATE_IDLE || m_modemState == STATE_M17)
+                err = m17TX.writeData(m_buffer + 3U, m_len - 3U);
+            }
+            if (err == 0U) {
+              if (m_modemState == STATE_IDLE)
+                setMode(STATE_M17);
+            } else {
+              DEBUG2("Received invalid M17 data", err);
+              sendNAK(err);
+            }
+            break;
+
           case MMDVM_POCSAG_DATA:
             if (m_pocsagEnable) {
               if (m_modemState == STATE_IDLE || m_modemState == STATE_POCSAG)
@@ -1196,6 +1237,46 @@ void CSerialPort::writeNXDNLost()
   writeInt(1U, reply, 3);
 }
 
+void CSerialPort::writeM17Data(const uint8_t* data, uint8_t length)
+{
+  if (m_modemState != STATE_M17 && m_modemState != STATE_IDLE)
+    return;
+
+  if (!m_m17Enable)
+    return;
+
+  uint8_t reply[130U];
+
+  reply[0U] = MMDVM_FRAME_START;
+  reply[1U] = 0U;
+  reply[2U] = MMDVM_M17_DATA;
+
+  uint8_t count = 3U;
+  for (uint8_t i = 0U; i < length; i++, count++)
+    reply[count] = data[i];
+
+  reply[1U] = count;
+
+  writeInt(1U, reply, count);
+}
+
+void CSerialPort::writeM17Lost()
+{
+  if (m_modemState != STATE_M17 && m_modemState != STATE_IDLE)
+    return;
+
+  if (!m_m17Enable)
+    return;
+
+  uint8_t reply[3U];
+
+  reply[0U] = MMDVM_FRAME_START;
+  reply[1U] = 3U;
+  reply[2U] = MMDVM_M17_LOST;
+
+  writeInt(1U, reply, 3);
+}
+
 void CSerialPort::writeCalData(const uint8_t* data, uint8_t length)
 {
   if (m_modemState != STATE_DSTARCAL)
diff --git a/SerialPort.h b/SerialPort.h
index cd09bbe..1e120c8 100644
--- a/SerialPort.h
+++ b/SerialPort.h
@@ -1,5 +1,5 @@
 /*
- *   Copyright (C) 2015,2016,2017,2018 by Jonathan Naylor G4KLX
+ *   Copyright (C) 2015,2016,2017,2018,2020 by Jonathan Naylor G4KLX
  *
  *   This program is free software; you can redistribute it and/or modify
  *   it under the terms of the GNU General Public License as published by
@@ -50,6 +50,9 @@ public:
   void writeNXDNData(const uint8_t* data, uint8_t length);
   void writeNXDNLost();
 
+  void writeM17Data(const uint8_t* data, uint8_t length);
+  void writeM17Lost();
+
   void writeCalData(const uint8_t* data, uint8_t length);
   void writeRSSIData(const uint8_t* data, uint8_t length);
 
diff --git a/Utils.cpp b/Utils.cpp
index 6a380f4..a51654a 100644
--- a/Utils.cpp
+++ b/Utils.cpp
@@ -30,6 +30,15 @@ uint8_t countBits8(uint8_t bits)
   return BITS_TABLE[bits];
 }
 
+uint8_t countBits16(uint16_t bits)
+{
+  uint8_t* p = (uint8_t*)&bits;
+  uint8_t n = 0U;
+  n += BITS_TABLE[p[0U]];
+  n += BITS_TABLE[p[1U]];
+  return n;
+}
+
 uint8_t countBits32(uint32_t bits)
 {
   uint8_t* p = (uint8_t*)&bits;
diff --git a/Utils.h b/Utils.h
index 95cb1d5..d8ebdd2 100644
--- a/Utils.h
+++ b/Utils.h
@@ -1,5 +1,5 @@
 /*
- *   Copyright (C) 2015,2016 by Jonathan Naylor G4KLX
+ *   Copyright (C) 2015,2016,2020 by Jonathan Naylor G4KLX
  *
  *   This program is free software; you can redistribute it and/or modify
  *   it under the terms of the GNU General Public License as published by
@@ -32,6 +32,8 @@
 
 uint8_t countBits8(uint8_t bits);
 
+uint8_t countBits16(uint16_t bits);
+
 uint8_t countBits32(uint32_t bits);
 
 uint8_t countBits64(uint64_t bits);