The following list summarizes the features that the library offers for Datamatrix barcodes. Some of the terms used here assumes familiarity with Datamatrix barcodes. All terms are also described in the remainder of this chapter.

This version of the library does not support the EDIFACT compaction standard due to the very specialized and limited use of this encodation schema.

Datamatrix as a standard is fully described in the ISO/IEC 16022E International Standard and is available for purchase from the ISO Standard Organization.

Additional information about Data Matrix code is available in the following United States patents: 4,939,354; 5,053,609; 5,124,536. See US patent Office for full disclosures of these patents.

Datamatrix is a two-dimensional symbology in the shape of a rectangle. The size and shape of the symbol is usually chosen either automatically or by the user. Usually it is chosen to be the smallest size that will have enough data capacity to encode the given data. The symbol rectangle is build up by square dots whose size "the module" is also user specified.

The Data Matrix symbol rectangle comes in two basic shapes.

Examples of the two basic shapes are shown in Figure 26.2 and Figure 26.3

Figure 26.2. Datamatrix - Square symbol shape

Figure 26.3. Datamatrix - Rectangle symbol shape

The maximum capacity for Data Matrix codes is up to 3116 numeric characters or up to 2335 alphanumeric characters or up to 1555 bytes of binary information.

The exact number of characters that can fit in a Data Matrix symbol depends on the actual encoding (or compaction) schema used. In short this is used to more efficiently encode ASCII characters to fit more data into a fixed number of bytes. For example if only numeric data is to be encoded then instead of using one byte to hold each digit two digits is stored in a single byte hence doubling the amount of data that can be stored in a given number of bytes.

To encode data into a Datamatrix symbol the following (principal) steps are taken.

The above explanation is by necessity simplified and for those interested into the specific details we refer to the official standard. It is also possible to review the code itself to understand the details.

As explained in the previous section several compaction schema are used to encode the data to enable more data to fit in a given symbol. Depending on the actual data there are several compaction schema that can be used in order to achieve the greatest possible compression. The standard specifies six different schema. The compaction efficiency are given in Table 26.1.

Depending on the application the user of the library may chose to either select a fixed encodation mode but it is usually best to let the library automatically select a combination of encodation schema that will give the smallest possible symbol size.

As was mentioned in the introduction there are two main subsets of Datamatrix symbols. Those using convolutional codes for error correction which were used for most of the initial installations of Datamatrix systems, these earlier versions are referenced as ECC-000 to ECC-140 (the number specifies the level of convolutional error correcting code).

This first subset will be commonly referred to as ECC-140 in the remainder of this manual.

The second subset is referenced ECC-200 and uses Reed-Solomon error correction techniques. The two subsets have the following characteristic:

Hence the type of encoding used is auto-discriminative. The maximum data capacity of an ECC-200 symbol is 3116 numeric digits, or 2335 alpha numeric characters, in the largest 144 modules square symbol.

Even though the library supports the creation of both type of Datamatrix symbols it is recommended that all new applications uses the more modern ECC-200 subset. This is also the recommendation in the standard. ECC-140 should only be used in legacy system where old equipment is used which have not be upgraded to handle the modern ECC-200 subset.

As was mentioned in the previous section the actual data capacity depends on the symbol size. By default the library will select the smallest possible symbol size that will encode a given character string with the chosen encoding (possibly automatic). Table 2 below gives the maximum capacity for the three most common encoding schema for each symbol size as well as robustness in each symbol specified as the number of errors (destroyed data) that can be recovered.