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2-D bar codes give us the ability to attach data files to physical objects.
By doing so, we can track items and information at the same time.

Bar Code Basics & PDF417 Symbology Overview:

All bar codes are comprised of bars and spaces (dark and light regions). A bar is a continuous dark area, a space is a continuous light area. From here on, I will refer to bar and/or space as simply bar. Most bar codes have a fixed number of possible bar widths. That is, each and every bar must be one of a fixed number of sizes. Each bar's width must also be a multiple of the smallest bar width. For instance, if the narrowest bar is 10 mil, then possible bar widths can be 10 mil, 20 mil, 30 mil, 40 mil, etc... But not, 15mil, or 25mil since these sizes are not multiples of the narrowest bar size of 10 mil.

X Dimension

The width of the smallest bar is defined as a bar code's 'X' dimension. Each X' dimension is sometimes referred to as a module. In the following picture, you can easily pick out the smallest, or narrowest bar. By measuring this bar, we can determine the bar codes 'X' dimension. You can also see that all of the other bar widths are multiples of the smallest bar or 'X' dimension. Each bar's width is often expressed relative to the 'X' dimension, for instance, 3X refers to a bar that is 3times as wide as the narrowest bar.

Like other barcodes, PDF417 has a well defined physical structure. This section describes each of these structural components.

Every bar code has a start pattern on the left, and a stop pattern on the right. These patterns are unique for each type of bar code. PDF417's unique start and stop patterns are:

In a PDF417 bar code, each bar and space does not store data. Data is actually stored in codewords. A codeword is a consecutive sequence of 4 bars and 4 spaces totaling 17X wide. The 417 in PDF417 refers to this codeword structure. Codewords reside between the start pattern on the left, and the stop pattern on the right. This region contains several types of codewords including data codewords, control codewords, and row indicator codewords. Each of these codewords abides by the rules described above. The picture below points out a single PDF417 codeword:

Adjacent to the start and stop patterns, are PDF's right and left row indicators. These indicators are codewords that are used to store information required to decode the PDF417 symbol. This includes the row number, number or rows, number of columns, and the error correction level used.

User data is first encoded into codeword values. These codeword values are then converted into physical codewords represented by bars and spaces as described above. Data codewords are physically located between the left and right row indicator codewords. Below, you can see the Data Codeword Region:

If you look closely at a PDF417 symbol, you will notice that it appears to be made of many "1D-like" bar codes. In reality, it is made up of multiple rows. A PDF417 bar code can have anywhere from 3 to 90 rows. This allows a PDF417 symbol to be reshaped by adjusting the number of rows. The following PDF417 symbol has 5 rows:

A PDF417 symbol is made up of multiple data columns, which are sometimes referred to as the data column area. The number of data columns can vary from 3 to 30, to accomadate user's real estate requirements. These columns contain encoded data, as well as error correction information. Within the data column area, a single PDF417 can contain no more than 928 codewords. An example of a 3 column PDF417 symbol appears below:

Error Correction

The PDF417 symbology has error correction capability. This capability enables scanners to read the bar code even if it has been torn, written on, or damaged in other ways. How much damage a symbol can withstand depends on the amount of error correction in each PDF417 symbol. The user has the ability to select 1 of 9 error correction levels for each symbol printed. Error correction is specified by selecting a level from 0 to 8. At level 0, a damaged PDF417 cannot be read, but the damage can be detected. At levels 1 through 8, a PDF417 symbol can still be read, even when damaged. As the error correction level increases, more damage can occur to the symbol and still be read. Consequently, the the higher the error correction level, the larger the symbol becomes, while the data capacity goes down. The following table illustrates these facts:

Error Correction Capacity

PDF417 symbols can be damaged and still decoded. The amount of damage that a symbol can withstand is it's error correction capacity. For example, at level 5 error correction, 64 codewords of error correction are used. At this level, 31 codewords can have errors, while still being read correctly. If more than 31 errors exist, the symbol is unreadable.

The Most Popular PDF417 and 2D Scanners

Metrologic 9544 2D Scanner

ImageTeam 3800PDF

 

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