Introduction

ISO 20794 (all parts) specifies the application (partly), application layer, transport layer, network layer, data link layer, and physical layer requirements of an in-vehicle network called "clock extension peripheral interface (CXPI)".

CXPI is an automotive low-speed single wire network. It is an enabler for reducing vehicle weight and fuel consumption by reducing wire counts to simple devices like switches and sensors.

CXPI serves as and is designed for automotive control applications, for example door control group, light switch, and HVAC (Heating Ventilation and Air Condition) systems.

The CXPI services, protocols, and their key characteristics are specified in different parts according to the OSI layers.

— Application and application layer:
- — application measurement and control data communication to exchange information between applications in different nodes based on message communication;
- — wake-up and sleep functionality;
- — two kinds of communication methods can be selected at system design by each node:
  1. i) the event-triggered method, which supports application measurement- and control-based (event-driven) slave node communication; and
  2. ii) the polling method, which supports slave node communication based on a periodic master schedule;
- — performs error detection and reports the result to the application;
- — application error management.
— Transport layer and network layer:
- — transforms a message into a single packet;
- — adds protocol control information for diagnostic and node configuration into each packet;
- — adds packet identifier for diagnostic and node configuration into each packet;
- — performs error detection and reports the result to higher OSI layers.
— Data link layer and physical layer:
- — provides long and short data frames;
- — adds a frame identifier into the frame;
- — adds frame information into the frame;
- — adds a cyclic redundancy check into the frame;
- — performs byte-wise arbitration and reports the arbitration result to higher OSI layers;
- — performs frame type detection in reception function;
- — performs error detection and reports the result to higher OSI layers;
- — performs Carrier Sense Multiple Access (CSMA);
- — performs Collision Resolution (CR);
- — generates a clock, which is transmitted with each bit to synchronise the connected nodes on the CXPI network;
- — supports bit rates up to 20 kbit/s.

To achieve this, it is based on the Open Systems Interconnection (OSI) Basic Reference Model specified in ISO/IEC 7498‑1 and ISO/IEC 10731^[²^], which structures communication systems into seven layers.

Figure 1 illustrates an overview of communication frameworks beyond the scope of this document including related standards:

— vehicle normal communication framework, which is composed of ISO 20794‑2 and this document;
— vehicle diagnostic communication framework, which is composed of ISO 14229‑1^[³^], ISO 14229‑2^[⁴^] and ISO 14229‑8^[⁵^];
— presentation layer standards, e.g. vehicle manufacturer specific or ISO 22901‑1 ODX^[¹⁰^];
— lower OSI layers framework, which is composed of ISO 20794‑3^[⁶^], ISO 20794‑4^[7], ISO 20794‑6^[⁸^] and ISO 20794‑7^[⁹^].

ISO 20794 (all parts) and ISO 14229‑8^[⁵^] are based on the conventions specified in the OSI Service Conventions (ISO/IEC 10731^[²^]) as they apply for all layers and the diagnostic services.

Figure 1 — ISO 20794 documents reference according to OSI model

BS ISO 20794-5:2020

Road vehicles. Clock extension peripheral interface (CXPI) - Application layer conformance test plan

Current, Under Review

Published:

30 Nov 2020