diff --git a/radio/src/cli.cpp b/radio/src/cli.cpp
index 54985b4df..bfafee823 100644
--- a/radio/src/cli.cpp
+++ b/radio/src/cli.cpp
@@ -874,30 +874,14 @@ int cliSet(const char **argv)
 }
 
 #if defined(ENABLE_SERIAL_PASSTHROUGH)
-// static StreamBufferHandle_t spIntRxBuffer;
-
-// static void spRx(uint8_t data)
-// {
-//   BaseType_t xHigherPriorityTaskWoken = pdFALSE;  
-//   xStreamBufferSendFromISR(spIntRxBuffer, &data, 1,
-//                            &xHigherPriorityTaskWoken);
-//   xTaskNotifyFromISR(cliTaskId.rtos_handle, 0, eNoAction,
-//                      &xHigherPriorityTaskWoken);
-
-//   // might effect a context switch on ISR exit
-//   portYIELD_FROM_ISR( xHigherPriorityTaskWoken );
-// }
-
 static void spInternalModuleTx(uint8_t* buf, uint32_t len)
 {
   intmoduleSendBuffer(buf, len);
   intmoduleWaitForTxCompleted();
 }
 
-static void spSerialTx(uint8_t data)
-{
-  serialPutc(data);
-}
+extern bool cdcConnected;
+uint32_t usbSerialFreeSpace();
 
 int cliSerialPassthrough(const char **argv)
 {
@@ -945,6 +929,9 @@ int cliSerialPassthrough(const char **argv)
     stopPulsesExternalModule();
 #endif
 
+    // suspend RTOS scheduler
+    vTaskSuspendAll();
+
 #if defined(HARDWARE_INTERNAL_MODULE)
     if (port_n == INTERNAL_MODULE) {
 
@@ -954,20 +941,31 @@ int cliSerialPassthrough(const char **argv)
       etx_serial_init params;
       params.baudrate = baudrate;
       params.rx_enable = true;
-      params.on_receive = spSerialTx;
-      intmoduleSerialStart(&params);
 
+      intmoduleSerialStart(&params);
       cliReceiveCallBack = spInternalModuleTx;
-      
-      // loop forever
-      for (;;) {
+
+      // loop until cable disconnected
+      while (cdcConnected) {
+
+        uint8_t data;
+        if (intmoduleFifo.pop(data)) {
+          uint8_t timeout = 10; // 10 ms
+          while(!usbSerialFreeSpace() && (timeout > 0)) {
+            delay_ms(1);
+            timeout--;
+          }
+
+          usbSerialPutc(data);
+        }
+
+        // RTOS_WAIT_MS(200);
         
         // keep us up & running
         WDG_RESET();
-        RTOS_WAIT_MS(200);
       }
 
-      intmoduleSerialStop();
+      intmoduleStop();
       // TODO: reset cli rx callback
     }
 #endif
@@ -975,6 +973,12 @@ int cliSerialPassthrough(const char **argv)
     // TODO:
     //  - external module (S.PORT?)
     //  - how to exit this mode ??? USB exit ?
+
+    watchdogSuspend(200/*2s*/);
+    resumePulses();
+
+    // suspend RTOS scheduler
+    xTaskResumeAll();
     
   } else {
     serialPrint("%s: invalid port type '%s'", port_type);
diff --git a/radio/src/targets/common/arm/stm32/usbd_cdc.cpp b/radio/src/targets/common/arm/stm32/usbd_cdc.cpp
index b8f48581a..332600e5d 100644
--- a/radio/src/targets/common/arm/stm32/usbd_cdc.cpp
+++ b/radio/src/targets/common/arm/stm32/usbd_cdc.cpp
@@ -151,6 +151,21 @@ uint16_t usbWraps = 0;
 uint16_t charsWritten = 0;
 uint16_t usbTxErrors = 0;
 
+// return the bytes free in the circular buffer
+uint32_t usbSerialFreeSpace()
+{
+  // functionally equivalent to:
+  //
+  //      (APP_Rx_ptr_out > APP_Rx_ptr_in ? APP_Rx_ptr_out - APP_Rx_ptr_in :
+  //      APP_RX_DATA_SIZE - APP_Rx_ptr_in + APP_Rx_ptr_in)
+  //
+  //  but without the impact of the condition check.
+
+  return ((APP_Rx_ptr_out - APP_Rx_ptr_in) +
+          (-((int)(APP_Rx_ptr_out <= APP_Rx_ptr_in)) & APP_RX_DATA_SIZE)) -
+         1;
+}
+
 void usbSerialPutc(uint8_t c)
 {
   /*
@@ -172,14 +187,6 @@ void usbSerialPutc(uint8_t c)
     txDataLen -= APP_RX_DATA_SIZE;
   }
 
-  // buffer is too full, skip this write
-  if (txDataLen > (APP_RX_DATA_SIZE - CDC_DATA_MAX_PACKET_SIZE)) {
-    usbTxErrors++;
-    return;
-  }
-
-  ++charsWritten;
-
   /*
     APP_Rx_Buffer and associated variables must be modified
     atomically, because they are used from the interrupt
diff --git a/radio/src/targets/common/arm/stm32/usbd_conf.h b/radio/src/targets/common/arm/stm32/usbd_conf.h
index 28e830242..39b663734 100644
--- a/radio/src/targets/common/arm/stm32/usbd_conf.h
+++ b/radio/src/targets/common/arm/stm32/usbd_conf.h
@@ -59,8 +59,13 @@
 #define CDC_DATA_MAX_PACKET_SIZE     64   /* Endpoint IN & OUT Packet size */
 #define CDC_CMD_PACKET_SZE           8    /* Control Endpoint Packet size */
 
+#if defined(STM32F2)
 #define CDC_IN_FRAME_INTERVAL        4    /* Number of frames between IN transfers */
 #define APP_RX_DATA_SIZE             512  /* Total size of IN buffer: APP_RX_DATA_SIZE*8/MAX_BAUDARATE*1000 should be > CDC_IN_FRAME_INTERVAL */
+#else
+#define CDC_IN_FRAME_INTERVAL        15    /* Number of frames between IN transfers */
+#define APP_RX_DATA_SIZE             2048  /* Total size of IN buffer: APP_RX_DATA_SIZE*8/MAX_BAUDARATE*1000 should be > CDC_IN_FRAME_INTERVAL */
+#endif
 #define APP_FOPS                     VCP_fops
 
 #endif // _USBD_CONF_H_
diff --git a/radio/src/thirdparty/STM32_USB-Host-Device_Lib_V2.2.0/Libraries/STM32_USB_Device_Library/Class/cdc/src/usbd_cdc_core.c b/radio/src/thirdparty/STM32_USB-Host-Device_Lib_V2.2.0/Libraries/STM32_USB_Device_Library/Class/cdc/src/usbd_cdc_core.c
index 87fbf3fd0..5df0ac77f 100644
--- a/radio/src/thirdparty/STM32_USB-Host-Device_Lib_V2.2.0/Libraries/STM32_USB_Device_Library/Class/cdc/src/usbd_cdc_core.c
+++ b/radio/src/thirdparty/STM32_USB-Host-Device_Lib_V2.2.0/Libraries/STM32_USB_Device_Library/Class/cdc/src/usbd_cdc_core.c
@@ -186,14 +186,14 @@ __ALIGN_BEGIN uint8_t APP_Rx_Buffer   [APP_RX_DATA_SIZE] __ALIGN_END ;
 #endif /* USB_OTG_HS_INTERNAL_DMA_ENABLED */
 __ALIGN_BEGIN uint8_t CmdBuff[CDC_CMD_PACKET_SZE] __ALIGN_END ;
 
-volatile uint32_t APP_Rx_ptr_in  = 0;	// modified by OpenTX
-volatile uint32_t APP_Rx_ptr_out = 0;	// modified by OpenTX
+uint32_t APP_Rx_ptr_in  = 0;
+uint32_t APP_Rx_ptr_out = 0;
 uint32_t APP_Rx_length  = 0;
 
 uint8_t  USB_Tx_State = USB_CDC_IDLE;
 
 static uint32_t cdcCmd = 0xFF;
-static uint8_t cdcLen = 0;   // modified by OpenTX
+static uint32_t cdcLen = 0;   // modified by OpenTX
 
 /* CDC interface class callbacks structure */
 const USBD_Class_cb_TypeDef  USBD_CDC_cb =   // modified by OpenTX