Overview
ISO 11898-4:2004 - "Road vehicles - Controller area network (CAN) - Part 4: Time-triggered communication" defines a time-triggered communication option for the Controller Area Network (CAN) used in road vehicles. It specifies the frame synchronisation entity (FSE) and the mechanisms needed to build a deterministic, cyclic communication schedule across Electronic Control Units (ECUs). Time-triggered CAN is a higher-level protocol layer that leaves the underlying CAN protocol unchanged while providing predictable message latency independent of bus load.
Key topics and technical requirements
- Frame Synchronisation Entity (FSE): Coordinates logical link and media access controls to implement time-triggered behaviour; each CAN controller in a time-triggered CAN network has an FSE.
- Reference messages and time masters: A time master periodically sends a reference message to start a basic cycle; reference messages synchronise and calibrate local node clocks.
- Basic cycle, matrix cycle and time windows: Communication is organised as basic cycles subdivided into time windows and transmission columns forming a system matrix. Messages may be assigned to:
- Exclusive time windows (no bus competition)
- Arbitrating time windows (shared slots with arbitration)
- Levels of implementation:
- Level 1: Supports cyclic message transfer (cyclic scheduling).
- Level 2: Extends Level 1 by providing a global time across the network (local time + offset).
- Timing and synchronisation features: Local time generation, Cycle_Time, Cycle_Count and mechanisms for synchronisation, external clock synchronisation, and compensation for time discontinuities.
- Fault tolerance and redundancy: Procedures for time master initialisation, substitution of alternative time masters, failure handling (message status count, master state) and shutdown.
- Visible interfaces: Configuration, application and optional interfaces to manage triggers, message objects and status reporting.
Applications and who uses this standard
ISO 11898-4 is targeted at professionals implementing deterministic communication on CAN networks in road vehicles, including:
- Automotive OEMs and Tier-1 suppliers designing ECU networks
- Embedded systems and RTOS developers requiring cyclic, time-triggered messaging
- CAN controller and tool vendors implementing FSE functionality
- System integrators and safety-critical application engineers who need predictable latency and composability in distributed control systems
Typical applications include distributed real-time control where schedule predictability and composability are required (powertrain control, chassis systems, advanced driver-assistance systems where deterministic timing is important).
Related standards
- ISO 11898-1: Data link layer and physical signalling (required reference)
- ISO 11898-2: High-speed medium access unit
- ISO 11898-3: Low-speed fault-tolerant, medium dependent interface
Keywords: ISO 11898-4, Time-triggered CAN, CAN, controller area network, ECU, frame synchronisation, time master, basic cycle, system matrix, Level 1, Level 2, global time.
Open preview (PDF)Road vehicles — Controller area network (CAN) — Part 4: Time-triggered communication