text: <user>
  Create a function block for controlling a conveyor zone in TwinCAT Structured Text. The function block should include logic for a drive motor and a stop mechanism.<|endoftext|>
  <assistant>
  FUNCTION_BLOCK FB_ConveyorZone
  VAR_INPUT
      bEnable         : BOOL;    // Enable signal for the conveyor zone
      bPalletPresent  : BOOL;    // Sensor input for pallet presence
      bDownstreamClear: BOOL;    // Signal indicating downstream zone is clear
  END_VAR
  
  VAR_OUTPUT
      bDriveMotor     : BOOL;    // Output to control the drive motor
      bStopMechanism  : BOOL;    // Output to control the stop mechanism
      bZoneOccupied   : BOOL;    // Indicator for zone occupation
      bError          : BOOL;    // Error flag
      nErrorID        : UDINT;   // Error ID
  END_VAR
  
  VAR
      tonStopDelay    : TON;     // Timer for stop mechanism delay
      tStopDelayTime  : TIME := T#500MS; // Delay time for stop mechanism
  END_VAR
  
  // Main logic
  IF bEnable THEN
      // Control drive motor
      bDriveMotor := bPalletPresent AND bDownstreamClear AND NOT bStopMechanism;
      
      // Control stop mechanism
      IF bPalletPresent AND NOT bDownstreamClear THEN
          tonStopDelay(IN := TRUE, PT := tStopDelayTime);
          IF tonStopDelay.Q THEN
              bStopMechanism := TRUE;
          END_IF
      ELSE
          tonStopDelay(IN := FALSE);
          bStopMechanism := FALSE;
      END_IF
      
      // Set zone occupied status
      bZoneOccupied := bPalletPresent;
      
      // Reset error
      bError := FALSE;
      nErrorID := 0;
  ELSE
      // Disable all outputs when not enabled
      bDriveMotor := FALSE;
      bStopMechanism := FALSE;
      bZoneOccupied := FALSE;
      
      // Set error if trying to operate while disabled
      IF bPalletPresent THEN
          bError := TRUE;
          nErrorID := 16#1001; // Custom error code for operation while disabled
      END_IF
  END_IF
  
  (* 
      Explanation:
      This function block controls a single conveyor zone with a drive motor and stop mechanism.
      It handles basic interlocking with downstream zones and includes error handling for
      attempted operation while disabled.
  
      Suggested TwinCAT 3 features to enhance this implementation:
      1. Use of Structured Error Handling (TRY-CATCH) for more robust error management.
      2. Implementation of a state machine using enumerated types for clearer zone status control.
      3. Integration with TwinCAT's motion control libraries for more advanced motor control.
  *)<|endoftext|>
  