Chapter 6: Encoding Mechanism - Bars and Spaces |
Summary Overview |
Code 39, often called 'Code 3 of 9,' derives its name from a simple yet elegant encoding principle: each character in the symbology is represented by exactly nine elements---five bars and four spaces---of which exactly three are wide and six are narrow. The pattern formed by the positions of these wide and narrow elements determines which character is encoded. This binary-like system, where wide elements represent a value of 1 and narrow elements a value of 0, creates a robust and self-checking barcode symbology that has proven remarkably durable since its invention in 1974. While its relatively low data density compared to newer symbologies like Code 128 limits its use in space-constrained applications, its simplicity, wide compatibility, and the absence of a mandatory check digit have made it the backbone of labeling systems across numerous industries. This chapter explores the technical characteristics of Code 39's encoding mechanism and demonstrates how these features translate into practical advantages and constraints across manufacturing, defense, healthcare, automotive, logistics, and other sectors. |
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The Anatomy of a Code 39 Character |
At its most fundamental level, a Code 39 barcode is a visual language composed of dark bars and light spaces. The interplay between these two elements---their widths and their arrangement---carries the information that barcode scanners decode into meaningful data. To understand why Code 39 has enjoyed such widespread adoption and continues to serve critical functions in industries ranging from automotive manufacturing to healthcare, one must first appreciate the elegance of its encoding mechanism. |
Every character in the Code 39 symbology consists of nine elements: five bars and four spaces. This is where the name 'Code 3 of 9' originates. Within these nine elements, exactly three are 'wide' and six are 'narrow.' The wide elements carry the binary information, while the narrow elements serve as the background against which the wide elements are distinguished . This 'three of nine' pattern is the defining characteristic of the symbology and the source of its robustness. |
The distinction between wide and narrow elements is not absolute but relative. A wide element is typically two to three times the width of a narrow element . This wide-to-narrow ratio, often denoted as 2:1 or 3:1, provides flexibility in printing. A 2:1 ratio produces more compact barcodes, while a 3:1 ratio creates larger, more forgiving barcodes that are easier for scanners to read, especially when printed on rough surfaces or at lower quality . |
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The Binary Logic of Wide and Narrow |
The encoding logic is straightforward: treat a narrow element as a binary 0 and a wide element as a binary 1. Each character, therefore, is represented by a nine-bit pattern containing exactly three 1s and six 0s. For example, the letter 'A' in Code 39 has a specific pattern of wide and narrow bars and spaces that distinguishes it from the letter 'B.' The decoder's job is to measure the relative widths of the elements as it scans across the barcode and map the resulting pattern to the corresponding character in the Code 39 character set. |
This simplicity is one of the symbology's greatest strengths. The encoding is deterministic and does not require complex lookup tables for the core 43 characters---each character maps to a unique pattern of wide and narrow elements . This means that even relatively simple scanner hardware can decode Code 39 barcodes, which has contributed significantly to the symbology's widespread adoption. |
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The Self-Checking Property |
One of the most important technical characteristics of Code 39 is its self-checking property. Because each character has exactly three wide elements out of nine, the pattern is heavily constrained. A single printing defect that causes a narrow bar to print too wide or a wide bar to print too narrow is unlikely to transform one valid character into another valid character . Instead, the resulting pattern will have an incorrect number of wide elements, causing the decoder to reject it rather than misinterpret it. |
This self-checking property means that Code 39 does not require a mandatory check digit to ensure data integrity, unlike many other barcode symbologies. The character set itself provides a degree of error detection . While an optional Modulo 43 check digit can be added for applications requiring an additional layer of security, it is not required by the specification . This is a significant practical advantage in many industries, as it simplifies the process of generating barcodes and integrating them into existing systems. |
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The Start and Stop Characters |
Every Code 39 barcode begins and ends with a special character known as the start/stop character. This character is commonly represented as an asterisk (*) in barcode fonts and human-readable text, though the asterisk is not actually one of the 43 encodable data characters . Its primary purpose is to tell the scanner where the barcode begins and ends. The start and stop characters are encoded with the same three-wide, six-narrow pattern as data characters, but they are not transmitted as part of the data . |
The start and stop characters are crucial for two reasons. First, they allow the scanner to determine the direction in which the barcode is being scanned. The asymmetry of the pattern means a scanner can identify the start and end of the barcode regardless of whether it is scanned left-to-right or right-to-left . Second, they ensure that the scanner does not accidentally interpret extraneous markings or white space as valid data. The quiet zones---the blank spaces on either side of the barcode---must be at least ten times the width of a narrow element to prevent the scanner from misreading the surrounding area . |
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Technical Characteristics and Their Practical Implications |
The encoding mechanism of Code 39 has several technical characteristics that directly impact how and where the symbology is used. Understanding these characteristics is essential for appreciating its suitability for specific applications. |
Data Density and Space Requirements |
The most significant limitation of Code 39 is its low data density. Because each character requires nine elements, and because there must be a small inter-character gap (at least one narrow-element width) between characters, Code 39 barcodes are relatively long compared to other symbologies . A 10-character Code 39 barcode is approximately 40% wider than an equivalent Code 128 barcode encoding the same data . This means that Code 39 is not suitable for applications where space is extremely limited, such as small consumer goods or items with very small labels. |
However, this limitation is not necessarily a drawback in many industrial contexts. Manufacturing plants, warehouses, and military facilities typically have ample space on equipment, parts, and packaging to accommodate larger barcodes. In these environments, the robustness and simplicity of Code 39 often outweigh its space requirements. The ability to print Code 39 barcodes using standard printer fonts, without needing specialized barcode generation software, also makes it an attractive option for organizations that need to implement barcoding quickly and cost-effectively . |
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Variable Length and Character Set |
Code 39 is a variable-length symbology, meaning that the number of characters encoded can vary from one barcode to the next . This flexibility allows it to encode labels of different lengths without requiring padding or fixed-length formats. The character set includes 43 characters: uppercase letters A through Z, digits 0 through 9, and seven special characters: space, period, dash, dollar sign, slash, plus sign, and percent . This alphanumeric capability was revolutionary when Code 39 was introduced in 1974, as previous barcode symbologies were largely numeric-only . |
The character set does not include lowercase letters. However, an extension called Code 39 Full ASCII (also known as Code 39 Extended) allows the encoding of all 128 ASCII characters by using two-character combinations from the base character set . For example, a lowercase 'a' is encoded as '+A.' While this provides full ASCII support, it comes at the cost of significantly reduced data density---characters that are not in the base set require two Code 39 characters, effectively doubling the length of the barcode . In practice, most industrial applications use the base Code 39 character set, as uppercase letters and numbers are sufficient for identifying parts, assets, and shipments. |
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No Mandatory Check Digit |
The absence of a mandatory check digit is both an advantage and a limitation. On the one hand, it simplifies barcode generation and reduces the length of the barcode. On the other hand, it means that Code 39 lacks the error detection capabilities of symbologies like Code 128, which have mandatory check digits . However, the self-checking property of Code 39---the fact that a single bar or space error cannot produce another valid character---provides a baseline level of error detection . |
For applications requiring enhanced security, the optional Modulo 43 check digit can be added. This check digit is calculated from the sum of the character values, modulo 43, and appended to the data before the stop character . Many industry standards, such as the U.S. Department of Defense's LOGMARS specification, mandate the use of the Modulo 43 check digit . The existence of this option means that Code 39 can be adapted to meet the security requirements of even the most demanding applications. |
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Wide Compatibility |
Perhaps Code 39's greatest strength is its near-universal compatibility. Virtually every barcode scanner and reader on the market can decode Code 39 barcodes . This is partly due to the symbology's age---it has been in use since 1974---and partly due to its simplicity. The straightforward encoding mechanism and the absence of complex checksum requirements make Code 39 easy to implement in both hardware and software. |
For organizations, this compatibility means that they can deploy Code 39 barcodes without worrying about whether their scanners, printers, or software can handle them. It also means that Code 39 barcodes can be read by a wide variety of devices, from expensive industrial scanners to inexpensive smartphone barcode reader applications. This ubiquity has made Code 39 the 'lingua franca' of industrial barcoding. |
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Industry Applications |
The technical characteristics of Code 39---its alphanumeric capability, self-checking property, wide compatibility, and simplicity---have made it the barcode of choice for numerous industries. While Code 39 has been supplanted by Code 128 and other more efficient symbologies in some applications, it remains deeply embedded in many sectors, particularly in the United States. |
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United States Department of Defense and Military Logistics |
Perhaps the most significant and enduring application of Code 39 is in the U.S. Department of Defense (DoD). In the early 1980s, the DoD adopted Code 39 as part of its LOGMARS (Logistics Applications of Automated Marking and Reading Symbols) program . This program mandated the use of Code 39 barcodes on all government property, including equipment, spare parts, and supplies. The military specification MIL-STD-1189 defined the requirements for Code 39 barcodes used in military logistics . |
The DoD's adoption of Code 39 was driven by several factors. The symbology's alphanumeric capability was essential for encoding part numbers, serial numbers, and other alphanumeric identifiers. The self-checking property was important for ensuring reliability in the field, where barcodes might be damaged or printed on less-than-ideal surfaces. The absence of a mandatory check digit---though sometimes overridden by the optional check digit requirement in some specifications---simplified barcode generation. And the wide compatibility of Code 39 meant that scanners from different manufacturers could read military barcodes without modification. |
The MIL-STD-1189 specification has since been superseded by ANSI/AIM BC1/1995, but Code 39 remains a core requirement for military labeling . Today, MIL-STD-130, which governs the identification of U.S. military property, includes Code 39 as a compliant symbology . The legacy of LOGMARS means that Code 39 barcodes are still present on military equipment and in military supply chains worldwide. |
The technical requirements of the military context shaped how Code 39 is used. Military specifications often require a wide-to-narrow ratio of 2.5:1 or 3:1, ensuring that the barcodes are robust enough to withstand harsh environments . They also mandate relatively large quiet zones and minimum X dimensions (the width of the narrowest element) to ensure readability . These specifications, while increasing the size of the barcode, significantly improve its reliability. |
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Automotive Industry and AIAG Standards |
The automotive industry is another major user of Code 39. The Automotive Industry Action Group (AIAG), a non-profit organization that develops standards for the automotive supply chain, has adopted Code 39 for part labeling and tracking . The AIAG B-1 specification defines the requirements for barcodes used on automotive parts and assemblies. |
Code 39 is used to encode vehicle identification numbers (VINs), part numbers, supplier codes, and other critical data on automotive components. The symbology's alphanumeric capability is essential for encoding VINs, which consist of 17 characters that include both letters and numbers. The self-checking property ensures that a single printing defect will not cause a scanner to misread a critical part number, which could have serious consequences in manufacturing or logistics. |
The automotive supply chain is complex and global, involving thousands of suppliers and manufacturers. Code 39's wide compatibility is a major advantage in this context. Suppliers can print Code 39 barcodes on their parts using standard barcode fonts and printers, and manufacturers can read these barcodes using off-the-shelf scanners . This interoperability reduces costs and simplifies supply chain management. |
One of the challenges in automotive applications is the need to label small components with barcodes. Code 39's relatively low data density can be a problem when encoding long part numbers or detailed data on small parts. In these cases, suppliers may use Code 128, which offers higher data density, but Code 39 remains the standard for many applications due to legacy requirements and compatibility . |
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Healthcare and the Health Industry Bar Code (HIBC) Standard |
The healthcare industry has also embraced Code 39, primarily through the Health Industry Bar Code (HIBC) standard. The HIBC standard, developed by the Health Industry Business Communications Council (HIBCC), is used for labeling medical devices, pharmaceutical products, and healthcare supplies . |
Code 39's alphanumeric capability is essential for encoding product identifiers, lot numbers, expiration dates, and other data that may include letters and numbers. The self-checking property is particularly important in healthcare, where a misread barcode could lead to medication errors or the use of incorrect medical devices. The absence of a mandatory check digit simplifies labeling, though many healthcare applications use the optional Modulo 43 check digit for added security . |
The healthcare industry also uses Code 39 Extended (Full ASCII) to encode characters that are not in the base character set . For example, the HIBC standard includes special characters and symbols that may be encoded using the two-character combinations defined by Code 39 Extended. While this increases the length of the barcode, it provides the full ASCII support required by some healthcare applications. |
The use of Code 39 in healthcare is not without challenges. The need to label small vials, syringes, and other small items sometimes makes Code 39's low data density a limitation. In addition, the healthcare industry is increasingly moving toward two-dimensional barcodes like Data Matrix and QR codes, which can encode large amounts of data in a small space. However, Code 39 continues to be widely used for healthcare labeling, particularly for larger items like medical devices and supply boxes. |
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Logistics, Warehousing, and Shipping |
In logistics and warehousing, Code 39 barcodes are used extensively for tracking inventory, managing shipments, and routing items through supply chains. The symbology's variable-length capability is useful for encoding different types of data, from short product codes to longer serial numbers. The wide compatibility of Code 39 means that logistics providers can use a single barcode format that can be read by scanners at different points in the supply chain . |
One specific application in shipping is the use of Code 39 on bills of lading and shipping labels . Code 39 barcodes are used to encode the pro number (pronum), a unique number assigned to each shipment. The barcode is printed on the bill of lading and used to track the shipment as it moves through the supply chain. The use of a font-based Code 39 barcode simplifies the printing process---the shipping software simply selects the Code 39 font and prints the barcode along with the human-readable text . |
The challenge in logistics applications is that Code 39 barcodes need to be readable under less-than-ideal conditions, such as when labels are partially damaged or when items are moving quickly on conveyor belts. The self-checking property and the ability to use a wide-to-narrow ratio of 3:1 make Code 39 reasonably robust in these conditions . However, the relatively large size of Code 39 barcodes can be a problem for small packages, where label space is limited. |
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Manufacturing and Industrial Asset Tracking |
In manufacturing and industrial environments, Code 39 is used to track equipment, tools, and raw materials. The symbology's robustness and reliability are critical in these contexts, where barcodes may be exposed to dirt, oil, vibration, and other harsh conditions . Code 39 barcodes are printed on asset tags, equipment labels, and inventory shelves, allowing workers to quickly identify and track items. |
The use of Code 39 in manufacturing often involves encoding asset IDs, serial numbers, and maintenance codes. The alphanumeric capability allows for flexible and meaningful labeling, such as using a combination of letters and numbers to denote the asset type, location, and acquisition date. The self-checking property ensures that a misread barcode will not result in an incorrect asset being recorded as located or serviced. |
Manufacturing environments also benefit from Code 39's compatibility with barcode fonts. Companies can print Code 39 barcodes using standard desktop printers and barcode fonts, without needing specialized barcode printing equipment. This is particularly useful for internal asset tracking, where labels may be generated on demand . |
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Postal Services and Government Applications |
While the Universal Postal Union recommends using Code 128 for postal applications, some postal services continue to use Code 39 . In the United States, Code 39 has been used for internal mail tracking and for certain government mail applications. The symbology's wide compatibility and the simplicity of barcode font-based generation make it a convenient choice for organizations that need to implement postal barcoding quickly. |
Government agencies beyond the military also use Code 39 for asset tracking and inventory management. The General Services Administration (GSA) and other federal agencies have adopted Code 39 for labeling government property. The symbology's alphanumeric capability, self-checking property, and widespread compatibility make it suitable for a wide range of government applications. |
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Library and Educational Institutions |
Libraries and educational institutions use Code 39 barcodes for tracking books, equipment, and other assets . The symbology's alphanumeric capability allows for encoding ISBN numbers, accession numbers, and other identifiers that include letters and numbers. The variable-length capability makes it easy to encode short library codes or longer serial numbers. |
The use of Code 39 in libraries highlights the symbology's accessibility. Libraries can implement barcoding with minimal investment, using barcode fonts and standard printers. The wide compatibility of Code 39 ensures that library barcodes can be read by handheld scanners, fixed scanners, and even smartphone apps, simplifying checkout and inventory management processes. |
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Code 39 in Context: Comparison with Other Symbologies |
To fully appreciate Code 39's role, it is helpful to compare it with other barcode symbologies. The primary competitor to Code 39 in many applications is Code 128, which was developed later and offers higher data density. Code 128 can encode the same data as Code 39 in approximately 30-40% less space . Code 128 also has a mandatory check digit, providing better error detection. For new applications, Code 128 is often preferred because of its efficiency and security. |
However, Code 39's advantages remain significant. Its self-checking property provides a level of robustness that does not require a separate check digit. Its simpler encoding scheme makes it easier to implement in software and hardware. Its wide compatibility with existing scanners and printers reduces costs and simplifies deployment. For many organizations, the benefits of sticking with Code 39 outweigh the efficiency gains of switching to Code 128. |
Another comparison is with Code 93, which is a more compact version of Code 39. Code 93 encodes data in a similar alphanumeric character set but with higher density. However, Code 93 has never achieved the same level of adoption as Code 39, partly because Code 39 was already entrenched by the time Code 93 was introduced . |
The introduction of two-dimensional barcodes like Data Matrix and QR codes has further changed the barcode landscape. These symbologies can encode large amounts of data in a small space and include powerful error correction capabilities. In applications where space is extremely limited or where a large amount of data must be encoded, 2D barcodes are the preferred choice. However, 1D barcodes like Code 39 remain widely used for applications where the amount of data is small and the simplicity of implementation and scanning is paramount. |
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Challenges and Limitations of Code 39 |
While Code 39 has proven remarkably durable, it is not without limitations. Understanding these limitations is essential for deciding when to use Code 39 and when to use a different symbology. |
Low Data Density |
The most significant limitation of Code 39 is its low data density. Because each character requires nine elements plus an inter-character gap, Code 39 barcodes are long relative to the amount of data they encode . A Code 39 barcode encoding the same data as a Code 128 barcode is about 40% longer . This means that Code 39 is not suitable for applications where label space is very limited, such as small consumer goods or tiny medical devices. In these cases, Code 128 or a 2D barcode is a better choice. |
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Limited Character Set |
The base Code 39 character set includes only uppercase letters, digits, and a limited set of special characters . It does not include lowercase letters or many special symbols. While Code 39 Extended can encode the full ASCII character set, it does so by using two-character combinations, which further reduces data density . For applications that require encoding a wide range of characters in a compact space, Code 128 is a better choice. |
Vulnerability to Print Quality Issues |
Like all 1D barcodes, Code 39 is vulnerable to print quality issues. If the wide-to-narrow ratio is not maintained during printing, or if ink spread blurs the edges of the bars, the barcode may become unreadable . While the self-checking property provides some resilience against these issues, severe print quality problems can still cause decoding failures. Proper quality control during printing is essential for ensuring reliable scanning. |
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Legacy System Lock-in |
In many industries, Code 39 is deeply embedded in legacy systems that would be costly to change. These systems include barcode scanners, printers, software, and databases that are configured to work with Code 39. Changing to a different symbology would require reconfiguring or replacing these systems, as well as relabeling existing items. For organizations with large installed bases of Code 39 systems, the cost of change is prohibitive, and they continue to use Code 39 even when a newer symbology might offer advantages. |
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The Future of Code 39 |
Despite its age and limitations, Code 39 is likely to remain in use for many years to come. The symbology's widespread adoption, compatibility, and simplicity give it a staying power that newer symbologies have not yet matched. In many industries, Code 39 is so deeply embedded in supply chains and manufacturing processes that it will be used for decades. |
However, the future of Code 39 is not entirely secure. In new applications, Code 128 and 2D barcodes are often preferred because of their higher data density and better error detection capabilities. As organizations upgrade their systems, they may choose to transition away from Code 39 to more efficient symbologies. The healthcare industry, for example, is increasingly moving toward 2D barcodes for medical device labeling, driven by regulatory requirements for more detailed product information. |
The development of the International Standardization Organization (ISO) specification ISO/IEC 16388, which defines Code 39, ensures that the symbology remains a recognized standard . This standardization helps maintain compatibility and ensures that Code 39 barcodes can continue to be used and decoded reliably. The existence of this standard is a testament to the symbology's importance and longevity. |
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Conclusion |
Code 39's encoding mechanism---five bars and four spaces, with exactly three wide elements per character---is a model of simplicity and robustness. The self-checking property, alphanumeric capability, and wide compatibility that derive from this encoding mechanism have made Code 39 a foundational technology in industries ranging from military logistics to automotive manufacturing, healthcare, and warehousing. While the symbology's low data density and limited character set are significant drawbacks, these limitations are often outweighed by the practical advantages of simplicity, compatibility, and established infrastructure. |
The diverse applications of Code 39 demonstrate how technical characteristics translate into real-world utility. In military logistics, the self-checking property ensures reliable identification of critical equipment. In automotive manufacturing, the alphanumeric capability allows for meaningful part identification. In healthcare, the compatibility with existing scanners simplifies implementation. In shipping and warehousing, the ability to print barcodes using standard fonts reduces costs and complexity. |
As technology evolves, Code 39 will likely coexist with newer symbologies, each serving different needs and applications. For applications requiring high data density or extensive character support, Code 128 and 2D barcodes are the preferred choices. For applications where simplicity, compatibility, and robustness are paramount, Code 39 remains an excellent choice. The symbology's enduring popularity is a testament to the effectiveness of its design and the continued relevance of its technical characteristics. |
cs@easiersoft.com