Overview
ISO/IEC 9549:1990 - "Information technology - Galvanic isolation of balanced interchange circuits" - defines a method for galvanic isolation of balanced signal interchange circuits using optocoupler integrated-circuit technology. The standard targets two-condition line coding (e.g., NRZ) and is intended to ensure safe, reliable data transmission where different mains supplies, ground potential differences, long cable runs or high electromagnetic interference could otherwise cause transmission errors or equipment damage.
Key topics and technical requirements
- Optocoupler-based isolation: Uses optoelectronic integrated circuits to separate the unbalanced (station) side from the balanced interchange side.
- Isolated generator and isolated receiver: Parameters and measurement methods are specified for both components (balanced outputs/inputs and unbalanced station interfaces).
- Compatibility: Electrical characteristics are made compatible with ISO 8482 (multipoint twisted-pair interconnections) and CCITT Recommendation V.11 (point-to-point balanced circuits), enabling interoperation with existing equipment.
- Performance limits and tests
- Data signalling rates supported: up to 2 Mbit/s (point-to-point) and 1 Mbit/s (multipoint); implementations may be optimized for ≤20 kbit/s.
- Cable length guidance: point-to-point up to 1 000 m, multipoint up to 500 m (twisted pair).
- High-impedance (inactive) leakage and noise requirements: specified limits for leakage currents (≤150 µA) and differential voltage in high‑impedance state (≤4 mV).
- Dynamic switching: generator output transitions specified to change from 10% to 90% of steady-state in