Chapter 5 Barcodes
5-10
Two-dimensional Barcodes
PDF 417 is a two-dimensional stacked barcode symbology capable of encoding over a
kilobyte of data per label. This is important for applications where a barcode must be
more than merely an identifier, an index to reference a database.
The portable data file approach is well suited to applications where it is impractical to
store item information in a database or where the database is not accessible when and
where the item's barcode is read. Because a PDF417 symbol can store so much data, item
data such as the content of a shipping manifest or equipment maintenance history can be
carried on the item, without requiring access to a remote database.
Encoding data into a PDF417 barcode is a two-step process. First, data is converted into
codeword values of 0-928, which represent the data. This is high-level encoding. Then
the values are physically represented by particular bar/space patterns, which is low-level
encoding. Decoding is the reverse process.
In addition, PDF417 is an error-correcting symbology designed for real-world applica-
tions where portions of labels can get destroyed in handling. It performs error correction
by making calculations, if necessary, to reconstruct undecoded or corrupted portions of
the symbol.
PDF417 Symbol Description
At first glance, a PDF417 symbol looks like a set of stacked barcodes. When we look
closer to analyze how the symbol is put together, there are several key elements. These
are rows, start patterns, stop patterns, codewords, and modules, whose definitions are
explained below.
module
The narrowest width of a bar or space in the barcode. All bars or spaces are multiples (up
to six times) of this width. The nominal unit of measure.
codeword
A single group of bars and spaces (or elements) representing one or more numbers, let-
ters, or other symbols (i.e., codeword values for the data to be encoded). Each PDF417
codeword contains four bars and four spaces, for a total of 17 module widths. Each code-
word starts with a bar and ends with a space. See figure below.
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