ISO/IEC 646:1991

Overview

ISO/IEC 646:1991 specifies a 7‑bit coded character set for information interchange. It defines a repertoire of 128 characters (control characters and graphic characters such as letters, digits and symbols) and their coded representations for use with alphabets of the Latin script. The standard sets out the structure of the code, options for national and application‑oriented variations, and an International Reference Version (IRV). It is intended for data processing, data communications and interchange across media (punched tape/cards, magnetic/optical media, transmission channels, etc.).

Key topics and requirements

• Code structure: The 7‑bit set comprises a CO (control) set of up to 32 control characters, the graphic SPACE (SP), a GO graphic set of up to 94 graphic characters, and DELETE (DEL).
• Code table notation: A 7‑bit code table has 128 positions arranged in 8 columns (0–7) and 16 rows (0–15). Positions are identified by notations of the form x/y, which map to the seven bits b7…b1.
• Character allocations:
  • Unique allocations for most graphic positions (e.g., letters, digits, common symbols).
  • Alternative allocations for some positions (e.g., NUMBER SIGN vs POUND SIGN; DOLLAR SIGN vs CURRENCY SIGN) to enable national/application variants.
• Conformance and implementation:
  • A CC‑data‑element (coded‑character‑data‑element) conforms if its coded representations comply with the standard and the adopted version is identified.
  • Devices must document how users supply and recognize characters; originating devices must transmit any sequence from the adopted version; receiving devices must correctly interpret and present characters.
• Code extension and control functions:
  • The standard permits use of control characters for code extension where the 7‑bit repertoire is insufficient; procedures are specified in ISO 2022.
  • Control‑function definitions are given in ISO/IEC 6429.
• Identification of versions: Guidance is provided to define and identify the IRV, national versions and application‑oriented versions.

Applications and who uses it

ISO/IEC 646 is relevant for:

• Legacy systems and embedded devices that rely on 7‑bit encodings.
• Developers and integrators working on data interchange, communications protocols and file formats that must interoperate with older or constrained systems.
• Device manufacturers (terminals, printers, telecommunication equipment) needing conformance documentation.
• Standards bodies, archivists and organizations implementing national variants or migration plans from legacy encodings.

Related standards

• ISO 2022 - techniques for code extension and escape sequences.
• ISO/IEC 6429 - control functions for 7‑bit and 8‑bit coded character sets.

Keywords: ISO/IEC 646, 7‑bit coded character set, information interchange, Latin script, control characters, graphic characters, IRV, code table, ISO 2022, ISO/IEC 6429.