diff --git a/CalM17.cpp b/CalM17.cpp
index 5d4b1f82..4c567f2e 100644
--- a/CalM17.cpp
+++ b/CalM17.cpp
@@ -33,7 +33,8 @@ const uint8_t PREAMBLE[] = {0x00U,
 	0x77U, 0x77U, 0x77U, 0x77U, 0x77U, 0x77U, 0x77U, 0x77U,
 	0x77U, 0x77U, 0x77U, 0x77U, 0x77U, 0x77U, 0x77U, 0x77U};
 
-CCalM17::CCalM17()
+CCalM17::CCalM17() :
+m_transmit(false)
 {
 }
 
@@ -41,6 +42,9 @@ void CCalM17::process()
 {
   m17TX.process();
 
+  if (!m_transmit)
+    return;
+
   uint16_t space = m17TX.getSpace();
   if (space < 2U)
     return;
@@ -48,5 +52,15 @@ void CCalM17::process()
   m17TX.writeData(PREAMBLE, M17_FRAME_LENGTH_BYTES + 1U);
 }
 
+uint8_t CCalM17::write(const uint8_t* data, uint16_t length)
+{
+  if (length != 1U)
+    return 4U;
+
+  m_transmit = data[0U] == 1U;
+
+  return 0U;
+}
+
 #endif
 
diff --git a/CalM17.h b/CalM17.h
index 47c47e14..bf8257ea 100644
--- a/CalM17.h
+++ b/CalM17.h
@@ -31,7 +31,10 @@ class CCalM17 {
 
   void process();
 
+  uint8_t write(const uint8_t* data, uint16_t length);
+
 private:
+  bool m_transmit;
 };
 
 #endif
diff --git a/SerialPort.cpp b/SerialPort.cpp
index 477db942..d552efa2 100644
--- a/SerialPort.cpp
+++ b/SerialPort.cpp
@@ -1109,7 +1109,7 @@ void CSerialPort::processMessage(uint8_t type, const uint8_t* buffer, uint16_t l
 #endif
 #if defined(MODE_M17)
       if (m_modemState == STATE_M17CAL)
-        err = 0U;
+        err = calM17.write(buffer, length);
 #endif
 #if defined(MODE_POCSAG)
       if (m_modemState == STATE_POCSAGCAL)