Overview - ISO 2629:1973 (Basic mode control procedures: Conversational information message transfer)
ISO 2629:1973 is a short International Standard that extends the basic mode control procedures described in ISO/R 1745 to support fast, conversational information transfer between two terminals while remaining in Phase 3 (information transfer). Published in 1973, it defines how two stations on a data link can reverse master/slave roles and exchange information messages in an inquiry/response style interaction without re-establishing the link.
Key topics and technical requirements
- Phase 3 extension: Allows reversal of information transfer direction (master/slave reversal) while remaining in Phase 3 of the basic mode.
- Conversation rules:
- An information message sent in the opposite direction may replace a positive acknowledgement (ACK) under controlled conditions.
- Replacement is permitted only when the received message is terminated by ETX (or DLE ETX), not ETB (DLE ETB).
- The opening character of the returned message (e.g., STX, SOH, DLE STX, DLE SOH) is treated as a positive acknowledgement.
- An information message must never substitute for a negative acknowledgement (NAK).
- Security and error control:
- Systems may use block checking and ACK/NAK supervisory sequences to preserve link integrity.
- Numbering schemes (forward and/or backward) are recommended to avoid duplication or loss when reversals occur.
- Recovery procedures:
- If no reply or an invalid reply is received, the sender should repeat the information message or send ENQ to request a retransmission.
- Repeated transmissions can cause block duplication; robust numbering is necessary to detect and correct duplicates.
- Applicability limits:
- Intended for terminals where an operator plays a role (conversational/inquiry-response systems).
- Systems that embed supervisory/control information within messages or rely on operator procedures are not covered.
Applications and who uses it
ISO 2629 is applicable to designers and implementers of legacy and constrained data-communication systems that require:
- Conversational, half-duplex or reversible data exchange without leaving an established information-transfer phase.
- Inquiry/response terminal interactions (e.g., remote terminals, transaction inquiry systems).
- Protocol designers implementing basic-mode control behavior consistent with ISO/R 1745.
Typical users: standards engineers, protocol implementers, telecommunications equipment designers, and system integrators working with archival or standards-compliant data link procedures.
Related standards
- ISO/R 1745 - Basic mode control procedures for data communication systems (the foundational standard extended by ISO 2629:1973).
