Overview
ISO/IEC 24740:2008 - Responsive Link (RL) - defines a real-time communications protocol and interface for parallel and distributed control. The standard specifies the communications functions corresponding to OSI layers 1–4 (physical, data link, network and transport) and is targeted at point-to-point, machine-level networks requiring deterministic, low-latency interactions. Key RL features include dual full‑duplex channels, fixed-size prioritized packets, packet overtaking, prioritized routing and hardware forward error correction.
Key topics and technical requirements
- OSI layers 1–4 coverage: Physical interface, data link framing and error control, network routing and packet formats, and transport-level priority handling.
- Physical layer: Point-to-point serial link, separate transmission paths for data and event streams, variable link speeds (see Annex C).
- Data link layer: Error correction (hardware forward error correction), CODEC and encoding methods (bit stuffing, NRZI noted in the standard), fixed frame formats, and automatic reconfiguration (plug-and-play).
- Network layer: Fixed-size packet formats with headers and trailers, priority levels, packet overtaking (higher-priority packets bypass lower-priority traffic at nodes), independent routing for data and event links, and priority-based routing tables.
- Transport layer: Priority replacement for packet acceleration/deceleration, multi-link support and stream data transmission.
- Packet sizes: 64‑byte data packets and 16‑byte event packets (fixed-size, prioritized).
- Real-time support: Designed to enable preemption-like behavior in communication to meet hard or soft real-time deadlines; includes informative annexes on real-time characteristics and scheduling (EDF, RM).
Applications and who uses it
Responsive Link is suited to systems that require deterministic, low-latency inter-node communication:
- Industrial automation and factory floor distributed controllers
- Robotics and humanoid systems with tightly coupled sensors/actuators
- Embedded systems and distributed real-time control (e.g., automotive subsystems)
- Network appliance designers building deterministic control planes or management backplanes
Users include systems architects, embedded engineers, network hardware designers, integrators of distributed control systems, and real-time software developers.
Practical benefits
- Predictable latency through prioritized packet handling and overtaking
- Reduced retransmission via hardware error correction for reliability in control loops
- Simplified QoS for time-critical “event” traffic separate from bulk data streams
- Plug-and-play support eases deployment and reconfiguration in modular machines
Related standards
- Mapped to the OSI reference model (layers 1–4).
- Prepared under ISO/IEC JTC 1; recommended for use alongside other ISO/IEC networking and real‑time system guidance when designing distributed control networks.
Keywords: ISO/IEC 24740, Responsive Link, real-time communications, prioritized routing, packet overtaking, fixed-size packets, hardware error correction, OSI layers 1‑4, distributed control.