ISO 1177:1985

Overview

ISO 1177:1985 is an international standard published by the International Organization for Standardization (ISO) that defines the character structure for serial-by-bit start/stop and synchronous character-oriented data transmission. This standard applies primarily to information processing systems using 7-bit and 8-bit coded character sets, including ISO 646 and ISO 4873, as well as their extensions per ISO 2022. It specifies bit sequencing, parity bit usage, and framing techniques essential for reliable data interchange across digital communication networks.

This standard is foundational for ensuring compatibility and interoperability between data terminal equipment (DTE) and data circuit-terminating equipment (DCE), especially in systems standardized under CCITT and IEC/ISO telecommunications recommendations.

Key Topics

• Character Structure
  ISO 1177 defines the sequencing of bits in each character for serial data transmission. It mandates the least significant bit (LSB) be transmitted first and clarifies bit assignments within 7-bit and 8-bit coded character sets.

• Bit Sequencing and Frames
  The standard requires data bits to be transmitted in order, with controls over start and stop elements to frame characters reliably:

  • Start elements correspond to Condition A (space)
  • Stop elements correspond to Condition Z (mark)

• Parity Bit Specification
  In 7-bit transmissions, an additional parity bit (the eighth bit) ensures error detection:

  • Start/stop asynchronous transmission uses even parity (number of one bits is even)
  • Synchronous transmission uses odd parity (number of one bits is odd)

• Start/Stop and Synchronous Operation
  It specifies timing details for start and stop signals and describes how the intervals between characters should be handled in asynchronous and synchronous systems.

• Error Handling and DTE Reactions
  The standard recommends methods for detecting parity errors and suggests how equipment should signal or record these errors, such as using special control characters or graphical representations (e.g., reverse question mark, control letter SB).

• Unit Interval Timing
  For electromechanical devices operating up to 200 bps, the stop element duration should be two unit intervals, but more flexible timing is allowed under mutual agreement, especially for leased circuits.

Applications

ISO 1177:1985 applies to a broad range of data communication scenarios, including:

• Serial Data Communication
  Establishes rules for transmitting textual and control characters serially between devices in asynchronous (start/stop) or synchronous modes. It is vital in modem communication, point-to-point links, and network interfaces.

• Data Terminals and Modems
  The standard aids manufacturers and system designers in implementing consistent and interoperable interface protocols for terminals and modems that deploy 7-bit or 8-bit coded data.

• Telecommunications Infrastructure
  Supports compatibility across public switched telephone networks (PSTN) and leased line environments, ensuring accurate character framing and error detection for reliable message delivery.

• Legacy Systems and International Interchange
  Facilitates integration and international data exchange using standardized coded character sets and parity rules, supporting multilingual telecommunications and computing systems.

Related Standards

• ISO 646 – 7-bit coded character set for information interchange, specifying the character repertoire used in ISO 1177.
• ISO 4873 – 8-bit coded character set defining the character structure for extended data representation.
• ISO 2022 – Code extension techniques for 7-bit and 8-bit character sets, allowing multiple coded sets.
• ISO 2047 – Graphical representation for control characters within the 7-bit coded character set.
• CCITT Recommendations (ITU-T V and X Series) – Telecommunications interface and data transmission protocols aligning with ISO 1177 for terminal and circuit equipment signaling.

Keywords: ISO 1177, character structure, start/stop transmission, synchronous transmission, parity bit, serial communication, 7-bit code, 8-bit code, data terminal equipment, data communication standard, international data interchange, digital transmission, character-oriented transmission, telecommunications standard.