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Scanner Implementation

  • Article

2/27/2008

A scanner device is used to read bar code data.

The scanner is an exclusive-use device, as follows:

  • The application must claim the device before enabling it.
  • The application must claim and enable the device before the device begins reading input.

The scanner object follows the general model for event-drive input:

  • When input is received from the device, a DataEvent event is queued using the Microsoft helper method, ScannerBase.GoodRead. If the device receives bad data, the Service Object may also queue an ErrorEvent event by calling ScannerBase.FailedRead.
  • If the PosCommon.AutoDisable property is set to true, the ScannerBase class will set the PosCommon.EnableDevice property to false. If your Service Object has implemented this method, it will need to disable the device as appropriate.

A queued DataEvent event will only be delivered to the application when the property ScannerBase.DataEventEnabled is set to true.

  • The UPOS specification requires that data from the incoming DataEvent must be copied into the corresponding properties before being delivered to the application. The method ScannerBase.PreFireEvent, which is called just before delivering the DataEvent to the application, meets this requirement by calling ScannerBase.DecodeScanDataLabel and ScannerBase.DecodeScanDataType if the DecodeData property is set to true. Usually you have to implement these methods in your Service Object.
  • Scanned data is placed into the Scanner.BaseScanData property. If the application has set the ScannerBase.DecodeData property to true, then data is decoded into the ScannerBase.ScanDataLabel and ScanDataType properties.
  • Before the DataEvent is delivered to the application, the property ScannerBase.DataEventEnabled is set to false. This prevents further DataEvents from being delivered to the application until it has finished processing the current one. The application sets ScannerBase.DataEventEnabled to true when it is ready to process incoming events.
  • The ScannerBasic.DataCount property may be read to obtain the total number of queued events.
  • All queued events may be deleted by calling the ScannerBasic.ClearInput method.

In This Section

  • Data Decoding
    Describes the code necessary to decode device-specific data.
  • Scanner Events
    Demonstrates how a scanner Service Object uses POS for .NET queuing to raise events to applications.