Auto ID and the Barcode

Bar codes, auto ID and portable data capture are common terms in this niche of the IT industry — and they can be confusing for newcomers. This guide explains what auto ID is, how bar codes work, and why human-readable characters are usually printed alongside them.

Auto ID covers a range of technologies that let computers automatically identify an item, usually by reading data directly from it. It has many application areas, and companies have developed technologies such as bar codes, RF (radio frequency) tags, magnetic stripes, smart cards, optical character recognition (OCR), and optical mark readers (OMR). Each technology has its own strengths and weaknesses. In most applications, product or asset identification is best achieved with one of two technologies — bar codes or RFID (short-range radio tags) — and these are the focus of this guide. We will also explain how data is transferred to and from the product, covering terms such as "keyboard wedge," "batch," and "RF data network."

A bar code represents characters (numbers and/or letters) in a printed form that can be read and decoded by suitable equipment. There are many different "types" of bar code — these types are called symbologies — along with a range of technologies for reading them. Because a bar code is simply a set of encoded characters, "reading" or decoding it makes those characters available to the host computer to which the decoder is attached. The structure of a bar code varies from symbology to symbology, but the diagram below shows the components of a typical bar code.

Start/Stop characters are special codes marking each end of the bar code. Different symbologies use different start and stop characters.

Check character is generated from the data in the bar code and appears within it. It serves as a verification check that the data has been correctly decoded. Different bar code types use different methods to calculate the check character; in some symbologies it is optional, while in others it is always present.

Quiet zones are blank areas to the left and right of the bar code.

Bearer bars (shown above) are horizontal bars printed across the top and bottom of some bar codes. They help prevent partial reads if the reader drifts off the top or bottom of the code. Bearer bars are usually required only for certain code types (most commonly Interleaved 2 of 5), since the start and stop characters in most bar codes already make them unnecessary. When a bar code is scanned (see fig. 2 below), the reader's optical elements convert the black and white bars into an analogue (non-digital) electrical signal that varies with the light and dark parts of the code. In effect, the scanning optics "see" the bar code as an electrical signal with a corresponding high/low pattern. Once the code has been converted into an electrical signal, the reader's decoding element converts that analogue signal into digital data the attached computer can understand. The decoder analyses and decodes the signal according to a pre-defined set of rules — correctly referred to as algorithms — determined by the symbology being decoded. So, provided the bar code has been properly printed and scanned, the decoder can apply the decode algorithm to output the encoded characters as data.

Printing the encoded characters below the bar code is simply common sense. If a bar code cannot be scanned for some reason — damage, poor print quality, or a scanner failure — a human-readable fallback lets the associated application allow the user to manually enter the characters or digits. Some symbology specifications explicitly require the numbers to be printed below the code. While there may be applications where this isn't required, desirable, or even possible, it is generally good practice to also represent the bar code in human-readable form.