diff --git a/src/class/cdc/cdc_device.c b/src/class/cdc/cdc_device.c
index 0f18144a..48e94a6e 100644
--- a/src/class/cdc/cdc_device.c
+++ b/src/class/cdc/cdc_device.c
@@ -75,22 +75,24 @@ typedef struct
 //--------------------------------------------------------------------+
 CFG_TUSB_MEM_SECTION static cdcd_interface_t _cdcd_itf[CFG_TUD_CDC] = { { 0 } };
 
+// TODO will be replaced by dcd_edpt_busy()
 bool pending_read_from_host;
-static void _prep_next_transaction(void) {
-    uint8_t const itf = 0;
-    cdcd_interface_t* p_cdc = &_cdcd_itf[itf];
+static void _prep_out_transaction (uint8_t itf)
+{
+  cdcd_interface_t* p_cdc = &_cdcd_itf[itf];
 
-    // skip if previous transfer not complete
-    if (pending_read_from_host) {
-        return;
-    }
+  // skip if previous transfer not complete
+  // dcd_edpt_busy() doesn't work, probably transfer is complete but not properly handled by the stack
+//  if ( dcd_edpt_busy(TUD_OPT_RHPORT, p_cdc->ep_out) ) return;
+  if (pending_read_from_host) return;
 
-    // Prepare for incoming data but only allow what we can store in the ring buffer.
-    uint16_t max_read = tu_fifo_remaining(&p_cdc->rx_ff);
-    if (max_read >= CFG_TUD_CDC_EPSIZE) {
-        dcd_edpt_xfer(TUD_OPT_RHPORT, p_cdc->ep_out, p_cdc->epout_buf, CFG_TUD_CDC_EPSIZE);
-        pending_read_from_host = true;
-    }
+  // Prepare for incoming data but only allow what we can store in the ring buffer.
+  uint16_t max_read = tu_fifo_remaining(&p_cdc->rx_ff);
+  if ( max_read >= CFG_TUD_CDC_EPSIZE )
+  {
+    dcd_edpt_xfer(TUD_OPT_RHPORT, p_cdc->ep_out, p_cdc->epout_buf, CFG_TUD_CDC_EPSIZE);
+    pending_read_from_host = true;
+  }
 }
 
 //--------------------------------------------------------------------+
@@ -129,18 +131,13 @@ uint32_t tud_cdc_n_available(uint8_t itf)
 char tud_cdc_n_read_char(uint8_t itf)
 {
   char ch;
-  if (!tu_fifo_read(&_cdcd_itf[itf].rx_ff, &ch)) {
-      ch = -1;
-  }
-
-  _prep_next_transaction();
-  return ch;
+  return tud_cdc_n_read(itf, &ch, 1) ? ch : (-1);
 }
 
 uint32_t tud_cdc_n_read(uint8_t itf, void* buffer, uint32_t bufsize)
 {
   uint32_t num_read = tu_fifo_read_n(&_cdcd_itf[itf].rx_ff, buffer, bufsize);
-  _prep_next_transaction();
+  _prep_out_transaction(itf);
   return num_read;
 }
 
@@ -153,7 +150,7 @@ char tud_cdc_n_peek(uint8_t itf, int pos)
 void tud_cdc_n_read_flush (uint8_t itf)
 {
   tu_fifo_clear(&_cdcd_itf[itf].rx_ff);
-  _prep_next_transaction();
+  _prep_out_transaction(itf);
 }
 
 //--------------------------------------------------------------------+
@@ -254,11 +251,12 @@ bool cdcd_open(uint8_t rhport, tusb_desc_interface_t const * itf_desc, uint16_t
 
   // Find available interface
   cdcd_interface_t * p_cdc = NULL;
-  for(uint8_t i=0; i<CFG_TUD_CDC; i++)
+  uint8_t cdc_id;
+  for(cdc_id=0; cdc_id<CFG_TUD_CDC; cdc_id++)
   {
-    if ( _cdcd_itf[i].ep_in == 0 )
+    if ( _cdcd_itf[cdc_id].ep_in == 0 )
     {
-      p_cdc = &_cdcd_itf[i];
+      p_cdc = &_cdcd_itf[cdc_id];
       break;
     }
   }
@@ -305,7 +303,7 @@ bool cdcd_open(uint8_t rhport, tusb_desc_interface_t const * itf_desc, uint16_t
 
   // Prepare for incoming data
   pending_read_from_host = false;
-  _prep_next_transaction();
+  _prep_out_transaction(cdc_id);
 
   return true;
 }
@@ -375,6 +373,7 @@ bool cdcd_control_request(uint8_t rhport, tusb_control_request_t const * request
 
 bool cdcd_xfer_cb(uint8_t rhport, uint8_t ep_addr, xfer_result_t result, uint32_t xferred_bytes)
 {
+  (void) rhport;
   (void) result;
 
   // TODO Support multiple interfaces
@@ -398,11 +397,12 @@ bool cdcd_xfer_cb(uint8_t rhport, uint8_t ep_addr, xfer_result_t result, uint32_
     // invoke receive callback (if there is still data)
     if (tud_cdc_rx_cb && tu_fifo_count(&p_cdc->rx_ff) ) tud_cdc_rx_cb(itf);
 
+    // prepare for OUT transaction
     pending_read_from_host = false;
-    _prep_next_transaction();
+    _prep_out_transaction(itf);
   }
 
-  // nothing to do with in and notif endpoint
+  // nothing to do with in and notif endpoint for now
 
   return true;
 }