ISO 20794-2:2020

Overview - ISO 20794-2:2020 (CXPI Part 2: Application layer)

ISO 20794-2:2020 defines the application layer protocol for the Clock eXtension Peripheral Interface (CXPI), an automotive low‑speed single‑wire in‑vehicle network. This part specifies how applications exchange measurement and control data, manage messages and faults, and interact with lower OSI layers via a defined service interface. CXPI is intended to reduce wiring (and vehicle weight) by connecting simple devices such as switches and sensors over a single wire.

Key topics and technical requirements

• Message structure and transfer

  • Defines application layer message formats and the A_PDU response field.
  • Specifies message types and PDU (protocol data unit) handling.

• Service interface and parameters (SI / SIP)

  • Standardizes the service interface between application and lower layers (A_Data.req / A_Data.ind).
  • Lists SIP parameters such as Mtype, ReqId, ReqTypeId, PDU, Length, SCT (sequence count), Result, NMInfo, ev_wakeup_ind, and cmd_wakeup_req.

• Communication methods

  • Supports two selectable node-level methods:
    • Event‑triggered (event-driven publisher/subscriber communication)
    • Polling (master‑controlled periodic schedule)

• Network management (optional)

  • Node states (normal, standby, sleep), wake‑up/sleep sequences and related parameters.
  • Optional NM information exchanged via SIP.

• Measurement and control data management

  • Publisher/subscriber model for measurement and control data.
  • Data types, consistency requirements, ReqId assignment and priority.

• Error handling and fault management

  • Application error handling, CXPI network error reporting, optional SCT error handling, retransmission and transmission prohibition mechanisms.
  • Error detection features to report faults to the application.

Practical applications and target users

• Typical CXPI use cases: door control modules, light switches, HVAC controls, sensors and simple actuators where wiring simplification and low cost are priorities.
• Who uses this standard:
  • Automotive system architects and OEMs designing body and comfort networks.
  • ECU and microcontroller firmware engineers implementing application‑layer CXPI protocols.
  • Suppliers of switches, sensors, gateways and diagnostic tools integrating CXPI communication.
  • Test and validation teams verifying message handling, wake/sleep behavior and error management.

Related standards

• Other parts of the ISO 20794 series (transport, network, data link and physical layers) - together they define the complete CXPI stack.
• References to OSI models: ISO/IEC 7498‑1 and ISO/IEC 10731 (basic reference for layered communication).

Keywords: ISO 20794-2, CXPI, application layer, clock extension peripheral interface, in‑vehicle network, event‑triggered, polling, service interface parameters, automotive wiring reduction, measurement and control data.