Overview
ISO/IEC 7480:1991 defines signal quality requirements for start‑stop (asynchronous) serial data transmission at DTE/DCE interfaces. It applies where the interface conforms to CCITT (now ITU‑T) recommendations such as V.24 and X.24 (telephone and data networks). The standard complements electrical interface characteristics in V‑series/X‑series recommendations by specifying measurable signal‑quality limits and performance categories for transmitting and receiving Data Terminal Equipment (DTE). It does not specify line/DCE signal quality, bit‑error rates or data speeds.
Key topics and technical requirements
- Scope and applicability
- Targets start‑stop transmission at DTE/DCE interfaces (asynchronous DCEs or synchronous DCEs operating in asynchronous mode per V.14).
- Applicable regardless of multiplexing, but not when intermediate equipment provides no signal regeneration.
- Timing definitions
- Differentiates mechanical timing (timing derived from mechanical/rotational sources) and electronic timing (timing derived from an electronic/isochronous signal).
- Performance categories
- Defines four performance categories for transmitters (two for mechanical timing, two for electronic timing) and complementary categories for receivers. These categories enable contract/test criteria between different vendors.
- Signal‑quality metrics
- Synchronous start‑stop distortion and gross start‑stop distortion - limits on timing displacement of signal transitions relative to theoretical instants.
- Minimum signal element duration - lower bound on element lengths as a percentage of unit interval.
- Character interval requirements - average and minimum allowed character durations (expressed relative to the unit interval).
- Modulation rate accuracy - allowable deviation of the modulation (baud) rate.
- Margins (synchronous, net, practical) - tolerated distortion margins compatible with correct decoding under defined conditions.
- Measurement guidance
- Measurements relate to the actual mean incoming modulation rate; Annex A is informational.
Practical applications and users
- Equipment designers and manufacturers of modems, terminal adapters, serial interface cards and other DTE/DCE devices use ISO/IEC 7480 to:
- Design transmit/receive timing and tolerances.
- Specify interoperability requirements when devices from different vendors interconnect.
- Test labs and conformance teams use the standard for acceptance testing and characterizing device behaviour on telephone/data networks.
- Network integrators and procurement engineers include these signal‑quality requirements in contracts for reliable asynchronous serial links.
Related standards
Normative and related documents referenced include ITU‑T (CCITT) V‑ and X‑series recommendations such as V.10, V.11, V.14, V.21, V.22, V.22bis, V.23, V.26ter, V.28, V.32, V.32bis, V.42, V.110, V.120, X.20, X.20bis, X.24, X.30 and ISDN reference configurations. These define electrical characteristics and signalling frameworks complementary to ISO/IEC 7480.