projector version

python_app/hardware_full/laser_control/exceptions.py

@@ -68,6 +68,24 @@ class PortNotFoundError(CommunicationError):
         super().__init__(message)
 
 
+class PortBusyError(CommunicationError):
+    """Raised when the serial port is already held by another program.
+
+    A laser controller is a single half-duplex request/response bus: only one
+    process may own it at a time. When the port is opened exclusively
+    (``exclusive=True``), a second exclusive open fails with this error instead
+    of silently sharing the bus and corrupting both sides' traffic.
+    """
+
+    def __init__(self, port: str = None):
+        target = f"'{port}'" if port else "the laser controller port"
+        super().__init__(
+            f"Serial port {target} is busy — another program is already using "
+            f"the laser controller. Close it first (only one program may own the "
+            f"port at a time)."
+        )
+
+
 class DeviceNotRespondingError(CommunicationError):
     """Exception raised when device doesn't respond to commands."""