Devicenet has become one of the most widely supported control networks. It is an open standard, so components from a variety of manufacturers can be used together in the same control system. It is supported and promoted by the Open Devicenet Vendors Association (ODVA) (see http://www.odva.org). This group includes members from all of the major controls manufacturers.

This network has been designed to be noise resistant and robust. One major change for the control engineer is that the PLC chassis can be eliminated and the network can be connected directly to the sensors and actuators. This will reduce the total amount of wiring by moving I/O points closer to the application point. This can also simplify the connection of complex devices, such as HMIs. Two way communications inputs and outputs allow diagnosis of network problems from the main controller.

Devicenet covers all seven layers of the OSI standard. The protocol has a limited number of network address, with very small data packets. But this also helps limit network traffic and ensure responsiveness. The length of the network cables will limit the maximum speed of the network. The basic features of are listed below.

· A single bus cable that delivers data and power.

· Up to 64 nodes on the network.

· Data packet size of 0-8 bytes.

· Lengths of 500m/250m/100m for speeds of 125kbps/250kbps/500kbps respectively.

· Devices can be added/removed while power is on.

· Based on the CANbus (Controller Area Network) protocol for OSI levels 1 and 2.

· Addressing includes peer-to-peer, multicast, master/slave, polling or change of state.

An example of a Devicenet network is shown in See A Devicenet Network. The dark black lines are the network cable. Terminators are required at the ends of the network cable to reduce electrical noise. In this case the PC would probably be running some sort of software based PLC program. The computer would have a card that can communicate with Devicenet devices. The FlexIO rack is a miniature rack that can hold various types of input and output modules. Power taps (or tees) split the signal to small side branches. In this case one of the taps connects a power supply, to provide the 24Vdc supply to the network. Another two taps are used to connect a smart sensor and another FlexIO rack. The Smart sensor uses power from the network, and contains enough logic so that it is one node on the network. The network uses thin trunk line and thick trunk line which may limit network performance.

A Devicenet Network