diff --git a/ADF7021.cpp b/ADF7021.cpp
index 7ef2c8d..65a4367 100644
--- a/ADF7021.cpp
+++ b/ADF7021.cpp
@@ -73,7 +73,13 @@ void CIO::ifConf()
   F_divider = floor(divider + 0.5);
 
   ADF7021_RX_REG0  = (uint32_t) 0b0000;
-  ADF7021_RX_REG0 |= (uint32_t) 0b01011   << 27;   // mux regulator/uart enabled/receive
+  
+#if defined(BIDIR_DATA_PIN)
+  ADF7021_RX_REG0 |= (uint32_t) 0b01001   << 27;   // mux regulator/receive
+#else
+  ADF7021_RX_REG0 |= (uint32_t) 0b01011   << 27;   // mux regulator/uart-spi enabled/receive
+#endif
+
   ADF7021_RX_REG0 |= (uint32_t) N_divider << 19;   // frequency;
   ADF7021_RX_REG0 |= (uint32_t) F_divider << 4;    // frequency;
   
@@ -84,7 +90,13 @@ void CIO::ifConf()
   F_divider = floor(divider + 0.5);
 
   ADF7021_TX_REG0  = (uint32_t) 0b0000;            // register 0
-  ADF7021_TX_REG0 |= (uint32_t) 0b01010   << 27;   // mux regulator/uart enabled/transmit
+
+#if defined(BIDIR_DATA_PIN)
+  ADF7021_TX_REG0 |= (uint32_t) 0b01000   << 27;   // mux regulator/transmit
+#else
+  ADF7021_TX_REG0 |= (uint32_t) 0b01010   << 27;   // mux regulator/uart-spi enabled/transmit
+#endif
+
   ADF7021_TX_REG0 |= (uint32_t) N_divider << 19;   // frequency;
   ADF7021_TX_REG0 |= (uint32_t) F_divider << 4;    // frequency;
 
@@ -235,9 +247,15 @@ void CIO::ifConf()
 //======================================================================================================================
 void CIO::setTX()
 { 
+  // Send register 0 for TX operation
   AD7021_control_byte = ADF7021_TX_REG0;         
   Send_AD7021_control();
   
+#if defined(BIDIR_DATA_PIN)
+  Data_dir_out(true);  // Data pin output mode
+#endif
+  
+  // PTT pin on
   PTT_pin(HIGH); 
   LED_pin(LOW);
 }
@@ -245,11 +263,18 @@ void CIO::setTX()
 //======================================================================================================================
 void CIO::setRX()
 { 
+  // Delay for TX latency
   delay_rx();
-    
+  
+  // Send register 0 for RX operation
   AD7021_control_byte = ADF7021_RX_REG0;
   Send_AD7021_control();
   
+#if defined(BIDIR_DATA_PIN)
+  Data_dir_out(false);  // Data pin input mode
+#endif
+  
+  // PTT pin off
   PTT_pin(LOW);
   LED_pin(HIGH);
 }
diff --git a/Config.h b/Config.h
index 559cde8..a0cf55f 100644
--- a/Config.h
+++ b/Config.h
@@ -22,6 +22,9 @@
 // Enable ADF7021 support:
 #define ENABLE_ADF7021
 
+// Bidirectional Data pin (Enable Standard TX/RX Data Interface of ADF7021)
+#define BIDIR_DATA_PIN
+
 // TCXO of the ADF7021:
 // For 14.7456 MHz:
 #define ADF7021_14_7456
diff --git a/IO.cpp b/IO.cpp
index d4676dc..8f2cffc 100644
--- a/IO.cpp
+++ b/IO.cpp
@@ -45,7 +45,10 @@ m_txBuffer(TX_RINGBUFFER_SIZE)
   COS_pin(LOW);
   DEB_pin(LOW);
 
+#if !defined(BIDIR_DATA_PIN)
   TXD_pin(LOW);
+#endif
+
   SCLK_pin(LOW);
   SDATA_pin(LOW);
   SLE_pin(LOW);
@@ -57,10 +60,8 @@ void CIO::process()
 
   // Switch off the transmitter if needed
   if (m_txBuffer.getData() == 0U && m_tx) {
-    DEB_pin(LOW);
     setRX();
     m_tx = false;
-    DEB_pin(LOW);
   }
 
   if (m_rxBuffer.getData() >= 1U) {
@@ -85,14 +86,19 @@ void CIO::interrupt()
     return;
 
   if(m_tx) {
-    DEB_pin(HIGH);
-
     m_txBuffer.get(bit);
 
+#if defined(BIDIR_DATA_PIN)
+    if(bit)
+      RXD_pin_write(HIGH);
+    else
+      RXD_pin_write(LOW);
+#else
     if(bit)
       TXD_pin(HIGH);
     else
       TXD_pin(LOW);
+#endif
 
   } else {
     if(RXD_pin())
diff --git a/IO.h b/IO.h
index 04c63a0..aef8442 100644
--- a/IO.h
+++ b/IO.h
@@ -46,6 +46,9 @@ public:
   void  SDATA_pin(bool on);
   void  SLE_pin(bool on);
   bool  RXD_pin();
+#if defined(BIDIR_DATA_PIN)
+  void  RXD_pin_write(bool on);
+#endif
   void  TXD_pin(bool on);
   void  PTT_pin(bool on);
   void  LED_pin(bool on);
@@ -57,6 +60,10 @@ public:
   void  COS_pin(bool on);
   void  interrupt(void);
 
+#if defined(BIDIR_DATA_PIN)
+  void  Data_dir_out(bool dir);
+#endif
+
   // IO API
   void  write(uint8_t* data, uint16_t length);
   uint16_t  getSpace() const;
diff --git a/IOArduino.cpp b/IOArduino.cpp
index eacc6ae..370bc94 100644
--- a/IOArduino.cpp
+++ b/IOArduino.cpp
@@ -32,7 +32,7 @@
 #define PIN_SLE        PB8
 #define PIN_RXD        PB4
 #define PIN_TXD        PB3
-#define PIN_TXRX_CLK   PA15
+#define PIN_CLKOUT     PA15
 #define PIN_LED        PC13
 #define PIN_DEB        PB9
 #define PIN_DSTAR_LED  PB12
@@ -51,7 +51,7 @@
 #define PIN_SLE        6
 #define PIN_RXD        7
 #define PIN_TXD        8
-#define PIN_TXRX_CLK   2   // 4 in Arduino Zero Pro
+#define PIN_CLKOUT     2   // 4 in Arduino Zero Pro
 #define PIN_LED       13
 #define PIN_DEB       11
 #define PIN_DSTAR_LED 14
@@ -92,8 +92,7 @@ void CIO::Init()
   pinMode(PIN_SDATA, OUTPUT);
   pinMode(PIN_SLE, OUTPUT);
   pinMode(PIN_RXD, INPUT);
-  pinMode(PIN_TXD, OUTPUT);
-  pinMode(PIN_TXRX_CLK, INPUT);
+  pinMode(PIN_CLKOUT, INPUT);
   pinMode(PIN_LED, OUTPUT);
   pinMode(PIN_DEB, OUTPUT);
   pinMode(PIN_DSTAR_LED, OUTPUT);
@@ -102,17 +101,47 @@ void CIO::Init()
   pinMode(PIN_P25_LED, OUTPUT);
   pinMode(PIN_PTT_LED, OUTPUT);
   pinMode(PIN_COS_LED, OUTPUT);
+  
+#if defined(BIDIR_DATA_PIN)
+  pinMode(PIN_TXD, INPUT);
+#else
+  pinMode(PIN_TXD, OUTPUT);
+#endif
 
 }
 
 void CIO::startInt()
 {
+#if defined(BIDIR_DATA_PIN)
+
+// TXD pin is TxRxCLK of ADF7021, standard TX/RX data interface
+#if defined (__STM32F1__)
+  attachInterrupt(PIN_TXD, EXT_IRQHandler, RISING);
+#else
+  attachInterrupt(digitalPinToInterrupt(PIN_TXD), EXT_IRQHandler, RISING);
+#endif
+
+#else
+
 #if defined (__STM32F1__)
-  attachInterrupt(PIN_TXRX_CLK, EXT_IRQHandler, RISING);
+  attachInterrupt(PIN_CLKOUT, EXT_IRQHandler, RISING);
 #else
-  attachInterrupt(digitalPinToInterrupt(PIN_TXRX_CLK), EXT_IRQHandler, RISING);
+  attachInterrupt(digitalPinToInterrupt(PIN_CLKOUT), EXT_IRQHandler, RISING);
 #endif
+
+#endif
+}
+
+#if defined(BIDIR_DATA_PIN)
+// RXD pin is bidirectional in standard interfaces
+void CIO::Data_dir_out(bool dir) 
+{
+  if(dir)
+    pinMode(PIN_RXD, OUTPUT);
+  else
+    pinMode(PIN_RXD, INPUT);
 }
+#endif
 
 void CIO::SCLK_pin(bool on) 
 {
@@ -134,6 +163,13 @@ bool CIO::RXD_pin()
   return digitalRead(PIN_RXD) == HIGH;
 }
 
+#if defined(BIDIR_DATA_PIN)
+void CIO::RXD_pin_write(bool on)
+{
+  digitalWrite(PIN_RXD, on ? HIGH : LOW);
+}
+#endif
+
 void CIO::TXD_pin(bool on) 
 {
   digitalWrite(PIN_TXD, on ? HIGH : LOW);
diff --git a/IOSTM.cpp b/IOSTM.cpp
index 11c43f6..4cc2e85 100644
--- a/IOSTM.cpp
+++ b/IOSTM.cpp
@@ -39,13 +39,18 @@
 #define PIN_RXD              GPIO_Pin_4
 #define PORT_RXD             GPIOB
 
+// TXD used in SPI Data mode of ADF7021
+// TXD is TxRxCLK of ADF7021, standard TX/RX data interface
 #define PIN_TXD              GPIO_Pin_3
 #define PORT_TXD             GPIOB
+#define PIN_TXD_INT          GPIO_PinSource3
+#define PORT_TXD_INT         GPIO_PortSourceGPIOB
 
-#define PIN_TXRX_CLK         GPIO_Pin_15
-#define PORT_TXRX_CLK        GPIOA
-#define PIN_TXRX_CLK_INT     GPIO_PinSource15
-#define PORT_TXRX_CLK_INT    GPIO_PortSourceGPIOA
+// CLKOUT used in SPI Data mode of ADF7021
+#define PIN_CLKOUT           GPIO_Pin_15
+#define PORT_CLKOUT          GPIOA
+#define PIN_CLKOUT_INT       GPIO_PinSource15
+#define PORT_CLKOUT_INT      GPIO_PortSourceGPIOA
 
 #define PIN_LED              GPIO_Pin_13
 #define PORT_LED             GPIOC
@@ -72,12 +77,21 @@
 #define PORT_COS_LED         GPIOB
 
 extern "C" {
+#if defined(BIDIR_DATA_PIN)
+  void EXTI3_IRQHandler(void) {
+    if(EXTI_GetITStatus(EXTI_Line3)!=RESET) {
+      io.interrupt();
+    EXTI_ClearITPendingBit(EXTI_Line3);
+    }
+  }
+#else
   void EXTI15_10_IRQHandler(void) {
     if(EXTI_GetITStatus(EXTI_Line15)!=RESET) {
       io.interrupt();
     EXTI_ClearITPendingBit(EXTI_Line15);
     }
   }
+#endif
 }
 
 /**
@@ -101,7 +115,11 @@ static inline void delay_us(uint32_t us) {
 }
 
 void CIO::delay_rx() {
+#if defined(BIDIR_DATA_PIN)
+  delay_us(290);
+#else
   delay_us(340);
+#endif
 }
 
 void CIO::dlybit(void)
@@ -163,16 +181,23 @@ void CIO::Init()
   GPIO_Init(PORT_RXD, &GPIO_InitStruct);
 
   // Pin TXD
+  // TXD is TxRxCLK of ADF7021, standard TX/RX data interface
   GPIO_InitStruct.GPIO_Speed = GPIO_Speed_50MHz;
   GPIO_InitStruct.GPIO_Pin   = PIN_TXD;
+#if defined(BIDIR_DATA_PIN)
+  GPIO_InitStruct.GPIO_Mode  = GPIO_Mode_IN_FLOATING;
+#else
   GPIO_InitStruct.GPIO_Mode  = GPIO_Mode_Out_PP;
+#endif
   GPIO_Init(PORT_TXD, &GPIO_InitStruct);
 
   // Pin TXRX_CLK
+#if !defined(BIDIR_DATA_PIN)
   GPIO_InitStruct.GPIO_Speed = GPIO_Speed_50MHz;
-  GPIO_InitStruct.GPIO_Pin   = PIN_TXRX_CLK;
+  GPIO_InitStruct.GPIO_Pin   = PIN_CLKOUT;
   GPIO_InitStruct.GPIO_Mode  = GPIO_Mode_IN_FLOATING;
-  GPIO_Init(PORT_TXRX_CLK, &GPIO_InitStruct);
+  GPIO_Init(PORT_CLKOUT, &GPIO_InitStruct);
+#endif
  
   // Pin LED
   GPIO_InitStruct.GPIO_Speed = GPIO_Speed_50MHz;
@@ -222,30 +247,60 @@ void CIO::Init()
   GPIO_InitStruct.GPIO_Mode  = GPIO_Mode_Out_PP;
   GPIO_Init(PORT_COS_LED, &GPIO_InitStruct);
 
+#if defined(BIDIR_DATA_PIN)
+  // Connect EXTI3 Line
+  GPIO_EXTILineConfig(PORT_TXD_INT, PIN_TXD_INT);
+  // Configure EXTI3 line
+  EXTI_InitStructure.EXTI_Line = EXTI_Line3;
+#else
   // Connect EXTI15 Line
-  GPIO_EXTILineConfig(PORT_TXRX_CLK_INT, PIN_TXRX_CLK_INT);
-
+  GPIO_EXTILineConfig(PORT_CLKOUT_INT, PIN_CLKOUT_INT);
   // Configure EXTI15 line
   EXTI_InitStructure.EXTI_Line = EXTI_Line15;
+#endif
+
   EXTI_InitStructure.EXTI_Mode = EXTI_Mode_Interrupt;
   EXTI_InitStructure.EXTI_Trigger = EXTI_Trigger_Rising;
   EXTI_InitStructure.EXTI_LineCmd = ENABLE;
   EXTI_Init(&EXTI_InitStructure);
-
 }
 
 void CIO::startInt()
 {
   NVIC_InitTypeDef NVIC_InitStructure;
-  
+
+#if defined(BIDIR_DATA_PIN)
+  // Enable and set EXTI3 Interrupt
+  NVIC_InitStructure.NVIC_IRQChannel = EXTI3_IRQn;
+#else
   // Enable and set EXTI15 Interrupt
   NVIC_InitStructure.NVIC_IRQChannel = EXTI15_10_IRQn;
+#endif
+
   NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
   NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
   NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
   NVIC_Init(&NVIC_InitStructure);
 }
 
+#if defined(BIDIR_DATA_PIN)
+// RXD pin is bidirectional in standard interfaces
+void CIO::Data_dir_out(bool dir) 
+{
+  GPIO_InitTypeDef GPIO_InitStruct;
+
+  GPIO_InitStruct.GPIO_Speed = GPIO_Speed_50MHz;
+  GPIO_InitStruct.GPIO_Pin   = PIN_RXD;
+  
+  if(dir)
+    GPIO_InitStruct.GPIO_Mode  = GPIO_Mode_Out_PP;
+  else
+    GPIO_InitStruct.GPIO_Mode  = GPIO_Mode_IN_FLOATING;
+  
+  GPIO_Init(PORT_RXD, &GPIO_InitStruct);
+}
+#endif
+
 void CIO::SCLK_pin(bool on)
 {
   GPIO_WriteBit(PORT_SCLK, PIN_SCLK, on ? Bit_SET : Bit_RESET);
@@ -266,6 +321,13 @@ bool CIO::RXD_pin()
   return GPIO_ReadInputDataBit(PORT_RXD, PIN_RXD) == Bit_SET;
 }
 
+#if defined(BIDIR_DATA_PIN)
+void CIO::RXD_pin_write(bool on)
+{
+  GPIO_WriteBit(PORT_RXD, PIN_RXD, on ? Bit_SET : Bit_RESET);
+}
+#endif
+
 void CIO::TXD_pin(bool on)
 {
   GPIO_WriteBit(PORT_TXD, PIN_TXD, on ? Bit_SET : Bit_RESET);