ISO 1745:1975

Overview

ISO 1745:1975, "Information processing - Basic mode control procedures for data communication systems," defines a foundational set of data link control rules for information interchange over a single data link using the ISO/CCITT 7‑bit coded character set (ISO 646). First published in 1975 (replacing Recommendation R 1745‑1971), the standard specifies message framing, supervisory sequences, transmission control characters, and phased link control for the basic mode of operation. It is intended as a minimal, extensible framework so devices of differing complexity can interoperate.

Key topics and technical requirements

• Transmission control characters: Defines ten control characters (e.g., SOH, STX, ETX, ETB, EOT, ENQ, ACK, NAK, DLE, SYN) and their basic meanings and usages for framing, supervision, and control.
• Message formats: Rules for headings and texts; framing with SOH/STX/ETX/ETB, optional block check characters (per ISO 1155), and use of EOT to abort or terminate transmissions.
• Supervisory sequences: Forward and backward supervisory sequences such as polling, selecting, identification/status requests, positive/negative acknowledgements, and disconnect procedures. Special DLE sequences are used for extended control functions (see annexes).
• Communication phases: A phased model covering establishment, data‑link setup, information transfer (with supervision), termination, and clearing - identifying which station is responsible in each phase.
• Operating assumptions & limits: Assumes single‑link operation, alternate mode transfer (turn‑around or separate channels), use of the 7‑bit ISO/CCITT code, and that one station handles automatic information exchange. Time‑out and recovery procedures are deferred to complementary standards.
• Extensibility and options: Minimal optional features to preserve interoperability; extensions are addressed in annexes and related ISO standards.

Practical applications

• Protocol design for low‑to‑medium complexity data terminals and data communication equipment.
• Implementing firmware/software for terminal adapters, modems, concentrators, store‑and‑forward switches, and speed‑change devices where basic data link control is required.
• Interoperability testing and conformity assessment when integrating equipment from different vendors that must support the basic mode.
• Legacy systems and environments that rely on ISO/CCITT character‑based control rather than modern byte/packet protocols.

Who should use this standard

• Telecom and data equipment manufacturers implementing basic link control.
• Systems integrators dealing with legacy or resource‑constrained devices.
• Test engineers and protocol implementers ensuring compliant framing and supervisory behavior.

Related standards

• ISO 1177 (start/stop & synchronous transmission character structure)
• ISO 1155 (block checking for information messages)
• ISO 2628 / ISO 2629 (basic mode complements and conventional message transfer)
• ISO 646 (ISO/CCITT 7‑bit coded character set)

Keywords: ISO 1745, basic mode control procedures, data communication systems, transmission control characters, message formats, supervisory sequences, data link control.