GE HE500TIU203, IC300TIU201, IC300TIU101, HE500TIU100 manual DeviceNet Overview, DeviceNet Features

PAGE 38	06 Apr 2000	CH. 8: NETWORKS
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"self-arbitration" causes the controller with the greater Network ID number to cease broadcasting without affecting the message-in-progress of the other controller.

In applications with a large number of networked controllers, better results may be achieved by assigning lower Network IDs to controllers that have more critical network data than other controllers. By assigning higher Network IDs to controllers that provide numerous network updates, the controllers are prevented from monopolising the bus time.

Each controller is capable of broadcasting Global Digital Output bits (%QG) and Global Analog Output bits (%AQG), which are periodically broadcasted to the other controllers on the network. The coil representations %QG and %AQG may be used in CBREEZE like any other coil or internal register reference.

All digital global outputs are broadcast to the network each time one of them has a state change. In addition, if a controller has not transmitted its global data for specific time period, the controller's programmable network timer may expire, which in turn results in a global data broadcast. Finally, as part of its power-up initialisation sequence, another controller can explicitly request a controller to broadcast its global data.

8.4DeviceNet Overview

DeviceNet is an open network. The specification and the protocol are open. Vendors are not required to purchase hardware, software or licensing rights to connect devices to a system.

8.4.1DeviceNet Features

DeviceNet is a low-cost communications link to connect industrial devices. It allows the interchangeability of simple devices while making interconnectivity of more complex devices possible. DeviceNet is based on CAN. It is an application layer protocol (ISO layer 7) and is defined in terms of an abstract object model, which represents the available communication services and the external visible behaviour of a DeviceNet node.

The DeviceNet Model is application independent. DeviceNet provides the communication services needed by various types of applications. Many of today's lower level industrial control devices must retain their low cost/low resource characteristics even when directly connected to a network. DeviceNet takes this into consideration by defining a specific instance of the DeviceNet Model for communications typically seen in a Master/Slave application. This is referred to as the Predefined Master/Slave Connection Set. Some of the features and functionality of the DeviceNet network are described Table 8.1.

Table 8.1 - DeviceNet Features and Functionality

Network Size		Up to 64 Nodes
Network Length	Selectable end-to end network distance varies with speed
	Baud Rate	Distance
	125 Kbps	500m	(1,640 feet)
	250 Kbps	250m	(820 feet)
	500 Kbps	100m	(328 feet)
Data Packets	0-8 bytes
Bus Topology	Linear (trunkline/dropline); power and signal on the same network cable
Bus Addressing	Peer-to-Peer with Multi-Cast (one-to-many); Multi-Master and Master/Slave
	special case; polled or change-of-state (exception-based)
System	Removal and replacement of devices from the network under power
Features