diff --git a/Config.h b/Config.h
index fd01960..9189105 100644
--- a/Config.h
+++ b/Config.h
@@ -21,6 +21,9 @@
 
 // #define  WANT_DEBUG
 
+// Allow the use of the COS line to lockout the modem
+// #define USE_COS_AS_LOCKOUT
+
 // For the original Arduino Due pin layout
 // #define  ARDUINO_DUE_PAPA
 
diff --git a/IO.cpp b/IO.cpp
index 65300a5..5ed3cdf 100644
--- a/IO.cpp
+++ b/IO.cpp
@@ -95,6 +95,7 @@ CIO::CIO() :
 m_pinPTT(PIN_PTT),
 m_pinCOSLED(PIN_COSLED),
 m_pinLED(LED1),
+m_pinCOS(PIN_COS),
 m_pinADC(PIN_ADC),
 m_pinDAC(PIN_DAC),
 m_ticker(),
@@ -114,7 +115,8 @@ m_ledValue(true),
 m_dcd(false),
 m_overflow(0U),
 m_overcount(0U),
-m_watchdog(0U)
+m_watchdog(0U),
+m_lockout(false)
 {
   ::memset(m_C4FSKState, 0x00U, 70U * sizeof(q15_t));
   ::memset(m_GMSKState,  0x00U, 40U * sizeof(q15_t));
@@ -132,6 +134,7 @@ m_watchdog(0U)
   pinMode(PIN_PTT,    OUTPUT);
   pinMode(PIN_COSLED, OUTPUT);
   pinMode(PIN_LED,    OUTPUT);
+  pinMode(PIN_COS,    INPUT);
 #endif
 }
 
@@ -232,6 +235,14 @@ void CIO::process()
     return;
   }
 
+#if defined(USE_COS_AS_LOCKOUT)
+#if defined(__MBED__)
+  m_lockout = m_pinCOS.read() == 1;
+#else
+  m_lockout = digitalRead(PIN_COS) == HIGH;
+#endif
+#endif
+
   // Switch off the transmitter if needed
   if (m_txBuffer.getData() == 0U && m_tx) {
     m_tx = false;
@@ -242,6 +253,19 @@ void CIO::process()
 #endif
   }
 
+  if (m_lockout) {
+    // Drain the receive queue
+    if (m_rxBuffer.getData() >= RX_BLOCK_SIZE) {
+      for (uint16_t i = 0U; i < RX_BLOCK_SIZE; i++) {
+        uint16_t sample;
+        uint8_t  control;
+        m_rxBuffer.get(sample, control);
+      }
+    }
+
+    return;
+  }
+
   if (m_rxBuffer.getData() >= RX_BLOCK_SIZE) {
     q15_t   samples[RX_BLOCK_SIZE];
     uint8_t control[RX_BLOCK_SIZE];
@@ -317,6 +341,9 @@ void CIO::write(q15_t* samples, uint16_t length, const uint8_t* control)
   if (!m_started)
     return;
 
+  if (m_lockout)
+    return;
+
   // Switch the transmitter on if needed
   if (!m_tx) {
     m_tx = true;
@@ -419,3 +446,8 @@ void CIO::resetWatchdog()
   m_watchdog = 0U;
 }
 
+bool CIO::hasLockout() const
+{
+  return m_lockout;
+}
+
diff --git a/IO.h b/IO.h
index 2ac77fd..962f6f4 100644
--- a/IO.h
+++ b/IO.h
@@ -46,6 +46,8 @@ public:
   bool hasTXOverflow();
   bool hasRXOverflow();
 
+  bool hasLockout() const;
+
   void resetWatchdog();
 
 private:
@@ -53,6 +55,7 @@ private:
   DigitalOut           m_pinPTT;
   DigitalOut           m_pinCOSLED;
   DigitalOut           m_pinLED;
+  DigitalIn            m_pinCOS;
 
   AnalogIn             m_pinADC;
   AnalogOut            m_pinDAC;
@@ -83,6 +86,8 @@ private:
   uint16_t             m_overcount;
 
   volatile uint32_t    m_watchdog;
+
+  bool                 m_lockout;
 };
 
 #endif
diff --git a/SerialPort.cpp b/SerialPort.cpp
index 251de85..f9bb4a2 100644
--- a/SerialPort.cpp
+++ b/SerialPort.cpp
@@ -57,7 +57,7 @@ const uint8_t MMDVM_DEBUG4       = 0xF4U;
 const uint8_t MMDVM_DEBUG5       = 0xF5U;
 const uint8_t MMDVM_SAMPLES      = 0xF8U;
 
-const uint8_t HARDWARE[]         = "MMDVM 20160229 (D-Star/DMR/System Fusion)";
+const uint8_t HARDWARE[]         = "MMDVM 20160303 (D-Star/DMR/System Fusion)";
 
 const uint8_t PROTOCOL_VERSION   = 1U;
 
@@ -129,6 +129,9 @@ void CSerialPort::getStatus() const
   if (io.hasTXOverflow())
     reply[5U] |= 0x08U;
 
+  if (io.hasLockout())
+    reply[5U] |= 0x10U;
+
   if (m_dstarEnable)
     reply[6U] = dstarTX.getSpace();
   else