check for usb busy

Src/syscalls.c

@@ -36,7 +36,6 @@ int _write(int file, char *data, int len) {
       if (len > UART_TX_BUFFER_SIZE)
         return 0;
       // Check if transfer is already ongoing
-      // TODO: use HAL_UART_GetState
       if(huart4.gState != HAL_UART_STATE_READY)
         return 0;
       // memcpy data into uart_tx_buf

Src/usbd_cdc_if.c

@@ -300,7 +300,10 @@ uint8_t CDC_Transmit_FS(uint8_t* Buf, uint16_t Len)
   //Check length
   if (Len > APP_TX_DATA_SIZE)
     return USBD_FAIL;
-  // TODO: check busy
+  // Check for ongoing transmission
+  USBD_CDC_HandleTypeDef* hcdc = (USBD_CDC_HandleTypeDef*) hUsbDeviceFS.pClassData;
+  if (hcdc->TxState != 0)
+    return USBD_BUSY;
   // memcpy Buf into UserTxBufferFS
   memcpy(UserTxBufferFS, Buf, Len);
   // Update Len