Code 128 Barcode Generator

Identifier structure

Code 128 is a high-density linear barcode for operational data such as serial numbers, asset tags, work orders and shipment references. It is not a retail GTIN symbol. The format has code sets: Set B represents printable ASCII characters, while Set C stores pairs of decimal digits as one codeword. This generator switches to Set C for sufficiently long even digit runs and returns to Set B for ordinary text.

Worked numerical example

The encoded stream starts with a set-specific start value. In a label such as BOX-2026-0042, letters and punctuation use Set B; the digit runs may use Set C in pairs 20, 26, 00 and 42. Every following codeword is multiplied by its one-based position, added to the start value, and reduced modulo 103 to form the mandatory checksum. A stop pattern finishes the symbol.

How the symbol encodes data

Auto-selection reduces width, but it does not change the original data. An odd-length digit run cannot be entirely paired, so one digit is emitted through a text set. A scanner reads codewords and reconstructs the exact character stream; it does not infer a product number, date or lot structure unless the application layer imposes one.

Physical failure mode

Readability problems often come from compression: narrow modules printed below device tolerance, bar-height reduction, thermal transfer darkness, damaged labels or a quiet zone invaded by a border. Code 128 needs a clear margin at both ends and an aspect ratio appropriate to the scanner. Do not use a screenshot as production art; download vector output and verify a real label.

Operational boundary

Code 128 differs from Code 39 because it supports a larger ASCII repertoire and can compact numeric pairs. It differs from EAN and UPC because it has no retail prefix and no fixed digit count. This page chooses code sets; the Code 39 page instead limits characters and offers its optional modulo-43 check character.

A format-specific proof

For BOX-2026-0042, the important proof is the exact returned character stream, including both hyphens and leading zeroes in numeric runs. A scanner that returns BOX20260042 may be configured to strip punctuation, or the host parser may be normalizing it; either result changes an asset key even if every bar scans. Code Set C is a compression choice inside the symbol, not an instruction for the receiving system to reinterpret paired digits.

Worked check-digit example

How the bars carry data

Printing and scanning limits

When to use this format

A parcel label where Set C saves width

Consider the internal reference BOX-2026-0042. The letters, hyphen and punctuation require Code Set B, while the four even-length numeric runs can use Set C as pairs: 20, 26, 00 and 42. Pairing does not turn the value into a number: the `00` remains two characters and must survive a scan into the warehouse field. The checksum is calculated from the actual start, data and switching codewords modulo 103, then followed by the stop pattern. ISO/IEC 15417 is the Code 128 symbology specification; GS1 notes that GS1-128 is a constrained subset distinguished by FNC1, not a synonym for arbitrary Code 128: https://ref.gs1.org/standards/genspecs/gscn/2020/GSCN_20-044_CodeSetC.pdf . This tool creates an ordinary Code 128 string, so it must not be used to fabricate a GS1-128 label merely by placing parentheses around a GTIN.

Test the receiving field as well as the bars

A useful trial prints BOX-2026-0042 at the final label width, scans it through the real handheld, and compares the received value byte for byte with the source. Repeat with BOX-2026-0043 and with BOX-2026-0002. The second catches a stale label; the third shows whether a leading pair of zeroes is retained. If the reader beeps but the host removes hyphens, changes case or interprets 0042 as a number, change the keyboard-wedge or parser rule rather than changing the barcode. Code 128 is appropriate for an internal token only when its data owner defines that token. It does not assign a retail number, define a lot or expiry grammar, or prove that a delivery partner will interpret its characters in the same field order. Preserve the source string, SVG revision, printer setting and scan result together; those four items separate an encoding fault from a thermal-print fault and an integration fault.

Keep business structure outside ordinary Code 128

If BOX-2026-0042 later needs a lot, date or serial rule, define those fields in the warehouse contract instead of relying on a scanner to infer them from punctuation. A plain Code 128 scan returns characters. It neither marks FNC1 nor supplies GS1 application-identifier semantics. Document field length, permitted characters and leading-zero treatment with the label specification, then reject any incoming value that violates that contract.

A controlled test string catches normalisation

Keep one test value with punctuation and a zero pair, such as BOX-2026-0042, in the commissioning procedure. It detects the three silent transformations that a simple all-digit label misses: punctuation stripping, conversion to a number, and loss of leading zeroes. A reader configuration is acceptable only if the host receives that exact reference string. Repeat the test after firmware updates, because parser defaults can change without changing the printed bars.