Decoder - The decoder in a barcode scanner performs a variety of functions. First, it analyses the digital signal from the sensor, and tests to see if it can be interpreted as a valid barcode. In this test, it looks for uniformity of the white space (high signal) on each side of the digital signal, and uniformity between the peaks and valleys of the digital signal itself. Then, it tests the digital signal for conformance with any and all of the barcode symbologies it's designed and set up to read.
When it finds a match for symbology, the decoder begins to test the digital signal for conformance with the barcode symbology format. This involves converting the peaks and valleys to ASCII text and doing the math required to calculate the check digits at the beginning and end of the barcode. If the signal passes all these tests, the decoder will do the whole process again until it's convinced that there is a valid barcode (the signal is valid) and that it makes sense.
Once the decoder has concluded that the barcode is valid and it has done the math to convert the digital signal to ASCII text, it checks the text formatting rules that it has been programmed with. For example, the decoder may be programmed with the default addition of a Carriage Return suffix at the end of the ASCII string. Many barcode scanner decoders have powerful and elegant text format abilities. For example, for a Code 39 barcode, it might be programmed to strip off the first 3 digits, add the character "A" at the beginning of the string, convert all the 7's to B's and add 5 Z's and a tab at the end. These text formatting capabilities can be programmed directly into the decoder using the set up manual to scan programming barcodes. Most scanner manufacturers also provide software utilities to perform the programming.
Then the text is formatted, it is sent to the host PC. If the connection is USB or Keyboard Wedge, the text will show up in the application where the cursor is flashing. If the connection is proprietary of RS232, the text will be pulled in by the application program and be stored appropriately. Using an RS232 connection gives you the ability to catch and process barcode data without the desktop PC user seeing anything on the screen.
Most often, the decoder is part of the barcode scanner itself, and the circuitry is normally located in the hand-grip of the hand-held barcode scanner. In older scanners, the decoder was too bulky to be included in the barcode scanner housing, so the scanner didn't have one. It sent the digital signal down the interface cable to a decoder box. The decoder box was connected to the host PC. Barcode scanners without an integrated decoder are called "undecoded" scanners.
In a nutshell, that's how a barcode scanner works. We've talked about corded barcode scanners and will look at them again, but this time in the context of all the other types of barcode scanners that are available.
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