Barcode Scanner Decoder

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.

• NOTE: The barcode scanner's decoder will be programmed to decipher some barcode symbologies but not others. It's important to look at the data sheet provided by the manufacturer of the scanner you are considering to make sure that it will decode the specific barcode symbology you need to scan. Scanners with good performance, price and durability are often only programmed to decode the most common of linear barcode symbologies.
• In most cases, you can tell the decoder to ignore barcodes that aren't of the symbology that you want. For example, on a shipping label, there are multiple barcodes in a relatively small space. Most of the time, they're of different symbology, and the scanner is set up to  only read 1 type during a particular operation. This keeps inappropriate data from being sent to the host computer.

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.

• NOTE: When you look at the data sheet for a barcode scanner, you'll see the "scan rate" specification. This number is generally between 70 and 200 scans per second. The decoder will be set by default to see 10-15 good reads of the barcode before it concludes that the barcode is valid and moves on to the step of formatting the ASCII text and sending it to the host.

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.