National Instruments NI-DNET Explicit Messaging Connections

Appendix A DeviceNet Overview

In Figure A-2, the term Application Object(s) refers to objects within the

device which are used to perform its fundamental behavior. For example,

within a photoelectric sensor, an instance of the Presence Sensing object

(an Application Object) represents the physical photoelectric sensor

hardware. Within a position controller device, an instance of the Position

Controller object (an Application Object) is provided for every axis (motor)

which can be controlled using the device.

For more information on the classes, instances, attributes, and services

provided by DeviceNet, refer to the DeviceNet Specification. You can find

additional information on the specific classes and instances supported by a

given device in the documentation that came with the device.

Although the NI-DNET driver software provides object instances which

are used to access the DeviceNet network, these objects do not correspond

directly to the objects defined by the DeviceNet Specification, and the

NI-DNET functions do not directly correspond to the services defined by

DeviceNet. To facilitate access to your DeviceNet network, the features

provided by the NI-DNET driver are a simplification of the objects and

services defined in the DeviceNet Specification.

Explicit Messaging Connections

Each device on the DeviceNet network supports at least one explicit

messaging connection. Explicit messaging connections provide a

general-purpose communication path used to execute services on a

particular object in a device.

For a given explicit messaging connection between two DeviceNet devices,

the device requesting execution of the service is called the client, and the

device to which the service request is directed is called the server. Your

NI-DNET software can be used as an explicit messaging client with any

number of DeviceNet server devices.

Using an explicit messaging connection, the client device sends an explicit

message request to the server device. This request indicates the service to

perform and the object to which the service is directed. When the server

receives the explicit message request, it executes the service and sends an

explicit message response to the client device. If the service executed

successfully, this response contains information requested by the client.

The MAC ID (address) of the explicit message client and server is

contained in the header of the DeviceNet explicit messages.