Overview
ISO 6936:1988 specifies rules for conversion between the CCITT International Telegraph Alphabet No. 2 (ITA 2) and the coded character sets of ISO 646 and ISO 6937‑2. The standard defines mappings for the 58 ITA 2 characters (including control functions) and provides conversion tables and conventions to enable interoperability between international telex services and data‑network terminals where the telex character repertoire is sufficient.
Key Topics
- Character mapping tables: Two principal tables specify ITA 2 → ISO 646/ISO 6937‑2 and ISO 646/ISO 6937‑2 → ITA 2 conversions (tables 1 and 2 in the standard).
- Scope of conversion: Covers 58 ITA 2 characteristics (alphabetic and control functions) and their equivalents in ISO 646 and ISO 6937‑2.
- Substitution rules:
- If no direct equivalent exists, characters are to be represented by a SUB graphic or the QUESTION MARK unless alternative conventions are agreed between parties.
- Non‑spacing characters in the supplementary set of ISO 6937‑2 must be removed from the character stream; other supplementary characters map to the QUESTION MARK.
- Positioning (letter/sign shift):
- ITA 2 uses positioning (LETTER / SIGN). When converting from ISO sets, the appropriate positioning codes must be inserted; if the current position is not the required one, the converter must prepend the necessary position character.
- The standard does not mandate lowercase vs uppercase for alphabetic ITA 2 representation; lowercase or uppercase may be used but mixing is prohibited.
- Control characters:
- Some control positions (for example positions 011, 012, 013, etc.) are generally not converted and may be removed by convention or equipment.
- National variants and annex:
- Annex A provides examples of replacement conversions and notes on national adaptations where character allocations differ.
Applications
ISO 6936 is practical for:
- Telex ↔ data network gateways and protocol converters that must translate between ITA 2 telex traffic and ISO‑based systems.
- Telecommunications equipment vendors implementing legacy telex support in modern terminals.
- Software developers and systems integrators handling archival message conversion, legacy system migration, or interoperability testing.
- Network operators ensuring accurate message exchange in international telex services.
Benefits include predictable character handling, reduced formatting problems when mapping limited telex repertoires to richer ISO character sets, and documented fallbacks for unmapped characters.
Related Standards
- ISO 646 - 7‑bit coded character set for information interchange
- ISO 6937‑2 - Coded graphic non‑alphabetic and diacritic characters for text transmission
- ISO 2022 - Techniques for extending 7/8‑bit coded sets
- CCITT/ITU‑T Recommendation F.1 (ITA 2), T.50, T.51 and S.18 - referenced telegraph/telex alphabet and conversion recommendations
Keywords: ISO 6936, ITA 2, ISO 646, ISO 6937‑2, telex conversion, character set conversion, telegraph alphabet, telex gateway, character encoding.