diff --git a/uhf_rfid_gui.py b/uhf_rfid_gui.py
index 74e6339..d67c883 100644
--- a/uhf_rfid_gui.py
+++ b/uhf_rfid_gui.py
@@ -256,6 +256,12 @@ class RfidApp(tk.Tk):
         self.stop_event = threading.Event()
         self.event_queue = queue.Queue()
         self.parser = FrameParser()
+        # No Windows, ler e escrever na porta serial ao mesmo tempo a partir
+        # de threads diferentes sem sincronizacao costuma gerar
+        # PermissionError/WinError 5 ("Acesso negado") em GetOverlappedResult
+        # ou WriteFile, dependendo do driver USB-serial. Este lock serializa
+        # todo acesso a self.ser (leitura e escrita) para evitar a corrida.
+        self._port_lock = threading.RLock()
 
         self.tags = {}  # epc -> {"count": int, "rssi": int, "last_seen": str}
 
@@ -409,9 +415,15 @@ class RfidApp(tk.Tk):
             return
 
         try:
-            self.ser = serial.Serial(port, baudrate=baud, timeout=0.2)
+            self.ser = serial.Serial(port, baudrate=baud, timeout=0.2, write_timeout=1.0)
         except Exception as exc:
-            messagebox.showerror("Erro ao conectar", str(exc))
+            messagebox.showerror(
+                "Erro ao conectar",
+                f"{exc}\n\n"
+                "Se o erro for 'Acesso negado'/PermissionError, verifique se a porta\n"
+                "nao esta aberta em outro programa (Monitor Serial da Arduino IDE,\n"
+                "outra instancia deste app, PuTTY, etc.) e tente novamente.",
+            )
             self.ser = None
             return
 
@@ -434,10 +446,11 @@ class RfidApp(tk.Tk):
         if self.reader_thread is not None:
             self.reader_thread.join(timeout=1.0)
         if self.ser is not None:
-            try:
-                self.ser.close()
-            except Exception:
-                pass
+            with self._port_lock:
+                try:
+                    self.ser.close()
+                except Exception:
+                    pass
         self.ser = None
         self._set_controls_connected(False)
         self._log("Desconectado")
@@ -448,32 +461,58 @@ class RfidApp(tk.Tk):
         finally:
             self.destroy()
 
+    def _fatal_disconnect(self, reason):
+        """
+        Encerra a conexao apos um erro irrecuperavel (ex.: PermissionError
+        do Windows) e devolve a interface a um estado consistente, para que
+        o usuario possa simplesmente clicar em "Conectar" de novo.
+        """
+        self._log(reason)
+        self.stop_event.set()
+        if self.ser is not None:
+            with self._port_lock:
+                try:
+                    self.ser.close()
+                except Exception:
+                    pass
+            self.ser = None
+        self._set_controls_connected(False)
+        self.status_var.set("Desconectado (erro)")
+
     # -------------------------------------------------------------- serial
     def _send(self, frame: bytes):
         if self.ser is None:
             return
         try:
-            self.ser.write(frame)
+            with self._port_lock:
+                self.ser.write(frame)
         except Exception as exc:
-            self._log(f"Erro ao enviar: {exc}")
+            self._fatal_disconnect(f"Erro ao enviar: {exc}")
             return
         self._log(f"TX: {frame.hex(' ').upper()}")
 
     def _reader_loop(self):
+        # Le em pequenos pedaços e mantem o lock so durante a chamada
+        # in_waiting+read (nao-bloqueante, pois so le o que ja esta
+        # disponivel), para nao competir por muito tempo com escritas
+        # feitas pela thread principal.
         while not self.stop_event.is_set():
             try:
-                if self.ser is None:
-                    break
-                waiting = self.ser.in_waiting
-                data = self.ser.read(waiting if waiting else 1)
+                with self._port_lock:
+                    if self.ser is None:
+                        break
+                    waiting = self.ser.in_waiting
+                    data = self.ser.read(waiting) if waiting else b""
             except Exception as exc:
-                self.event_queue.put(("log", f"Erro de leitura serial: {exc}"))
+                self.event_queue.put(("fatal", f"Erro de leitura serial: {exc}"))
                 break
 
             if data:
                 frames = self.parser.feed(data)
                 for frame in frames:
                     self.event_queue.put(("frame", frame))
+            else:
+                time.sleep(0.02)
 
     # ------------------------------------------------------------- actions
     def _set_power(self):
@@ -519,6 +558,8 @@ class RfidApp(tk.Tk):
                     self._log(payload)
                 elif kind == "frame":
                     self._handle_frame(payload)
+                elif kind == "fatal":
+                    self._fatal_disconnect(payload)
         except queue.Empty:
             pass
         finally: