RFID Systems - Allied Automation, Inc.

Barcode Imaging SystemsFundamentals of Barcode Imaging2-D “Barcodes”The need to store more information using barcodes resultedin the introduction of 2-D solutions. Starting in the late 1980s“stacked” codes were introduced. These codes are composedof a number of 1-D codes stacked above each other. Theresulting code is then placed between a common “Start” and“Stop” symbol. Later new symbologies were developed, mostof which require a “camera” based reader.Common 2-D SymbologiesCodablockTwo variants (A and F (basedon 128 barcode)) are used.This code allows up to 44individual code rows resultingin a maximum of 2725 data symbols. They are readable with“standard” line scanners. The common “Start” and “Stop”portion is seen in this example.MaxicodeMaxicode is a public domain 2-D symbologycurrently employed by UPS in packagesorting. The major drawback is its fixedphysical size (1” by 1”) and fixed data content.PDF 417PDF 417 is a compact code withhigh information density. The codeincludes Reed-Solomon forwarderror correction. It was originallydeveloped by Symbol Technologies.Data MatrixData Matrix is a 2-D symbology that hasgrown significantly over the last 5 years.The strength of Data Matrix is due to severalfactors that give this symbology significantadvantages in everyday applications:ScalabilityData Matrix code is fully scalable. As long as a readercan capture the image, and resolve the modules, anysize is possible. The two symbols on the right containthe exact same information, One is scaled to severalinches square. The smaller oneis less than 3/8” on each side.Smaller symbols are possibleas long as the printing processoffers enough resolution andthe “ink dots” remain welldefined on paper. Direct lasermarking can generate thesmallest symbols, as it doesnot involve any additionaltransfer processes. Laserprinters use a laser to transferthe image onto a printing drum.This changes the electrostatic properties of the drum, and theink powder sticks to the drum creating a positive of the image.The powder is then transferred to the paper and heated.Reading symbols over such a wide size range is possiblesince Data Matrix readers are not looking for edges (betweenwhite and black) and the width between the bars and whiteareas. Instead, the evaluation algorithm places a virtual gridover the recorded symbol and then looks for “dots” at the gridintersections. “Dots” are found by adding up the blacknessaround the grid intersections. This leads to fewer requirementson the print process and edge contrast is not critical.Consequently, many print methods can be used successfully.This is a goodtime to clarify themeaning of symbolsize versus symboldimension. Thesize of a symbol iscounted in modulesand determines howmuch informationcan be stored inthe symbol. Thesymbol dimensionis its actual size in1 2 3 4 5Symbol Size6 7 8 9 10ModuleSizemillimeters or inches. Given a certain symbol size (amount ofinformation) the resolution of the printing process sets a lowerlimit for the symbol dimension.Data Integrity & Forward Error CorrectionData Matrix ECC200 symbols are not scanned, but read viaan imaging system. Usually, the recorded image is evaluatedby a powerful DSP (Digital Signal Processor) or othermicroprocessor, and the resulting information is sent to anumber of different communication interfaces (RS-232, USB,etc.).Data Matrix Symbology employs Reed-Solomon errorcorrection. Forward error correction means that symbols canbe partially destroyed and the algorithm is able to correctlyread all stored information. This is possible since the data onthe symbol is stored with a certain amount of redundancy.Forward error correction is not a retry method. An image istaken once and decoded once. This error correction method isalso used on CDs and DVDs.The following example shows how humans perform justsuch forward error correction when reading text. Language(like Data Matrix codes) contains redundant information andthe brain can “fill in the blanks” as long as not too muchinformation is missing. An advertising campaign by a wellknownhotel chain read as follows:FR HI-SPD INT SNDS MSGS FASTRAfter looking at this for a bit, you will have no problemidentifying the real sentence to be:Free high-speed internet sends messages fasterCounting the number of letters and spaces in bothcases (29 and 46 respectively) we can say that the “fullsentence” is about 40% redundant. The error correctionalgorithm used with Data Matrix symbols is very powerfuland offers error correction rates that are approximatelyPepperl+Fuchs®Inc. • 1600 Enterprise Parkway • Twinsburg, Ohio 44087-2245 • www.am.pepperl-fuchs.com108Telephone (330) 486-0001 • FAX (330) 405-4710 • E-Mail: sales@us.pepperl-fuchs.com