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CAN APPLICATION LAYERS Controller Area Network - CAN Application Layers The OSI modelISO7498 defines a communications standard known as the Open Systems Interconnection (OSI) model. This model describes how communications should occur between computers on any network, and has been adopted as a general "open" network communication standard. In principle - anything that conforms to the standard can communicate, electronically, with anything else that conforms to the standard. The OSI model defines seven independent "layers" of a protocol stack. Strict compliance with the standard requires that each layer is insulated from the others by well-defined interfaces. Few, if any, networks comply absolutely with the OSI model with regard to provision of all seven layers as distinct entities. Fig 1. The Seven Layer OSI model Full details of the functionality of each layer are available in the ISO7498 standard. CAN and the OSI Model The CAN specification (ISO11898) discusses only the Physical and Data-Link layers for a CAN network. The Data-Link Layer - is the only layer that recognises and understands the format of messages. This layer constructs the messages to be sent to the Physical Layer, and decodes messages received from the Physical Layer. In CAN controllers, the Data-Link Layer is usually implemented in hardware.

The Physical Layer - specifies the physical and electrical characteristics of the bus, and of the hardware that converts the characters of a message into electrical signals for transmitted messages - and electrical signals into characters for received messages. Although the other layers may be implemented in either hardware (as chip level functions) or software, the Physical Layer is always "real" hardware.

CAN Applications LayersMany applications of CAN require services that are beyond the basic functionality specified by the Data-Link Layer but which may be implemented at the Application Layer. For example, the transmission or reception of data units longer than eight bytes. To meet this need several organisations have developed Application Layers. Brief details about just a few of them and contact details are given below. CAL (CAN Application Layer)Aptly named, and based on an existing and proven protocol originally developed by Philips Medical Systems, CAL is an application-independent application layer that has been specified and is now maintained by the CAN in Automation (CiA) user group. Anyone who implements CAL may do so free of any licence royalty. The CAL specification (document reference CiA DS-201...207) may be purchased from CiA. See the CiA web site for details.

CANopenCANopen is an implementation of CAL and is defined by the CANopen Communications Profile in CiA DS-301. This document may also be purchased from CiA. Information about CANopen may be obtained from the CiA User Group

PCALAnother example of a CAL implementation - known as P-CAL (Portable CAN Application Layer) - has been developed by the University of the German Federal Armed Forces in Munich, Germany. Licences are available for binary or source code, for a range of processors. For more information send e-mail to Nickolay Belofastow at the University.

DeviceNet™DeviceNet is a CiA-approved application layer based on CAN 2.0A and is widely used in industrial automation applications. DeviceNet (originally developed by Rockwell/Allen-Bradley) is now an "Open" fieldbus regulated by an independent organisation know as the Open DeviceNet Vendors Association, from whom copies of the specification may be purchased. Purchasers of the specification receive an unlimited, royalty-free licence to develop DeviceNet compatible products. See the ODVA web site for full details.

SDS™ (Smart Distributed System)SDS is also a CiA-approved application layer. Developed by Honeywell, one of the main uses of SDS is for machine control applications. See the Honeywell web site for details.

CAN Kingdom™Another CiA-approved application layer, named CAN Kingdom, is provided by a Swedish company named Kvaser AB. You can find out all about it if you click here

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