National Instruments NI-DNET - Manual (Page 52)

Appendix A DeviceNet Overview
© National Instruments Corporation A-5 NI-DNET User Manual
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.