Issue 98, September 1998
Networking with DeviceNet

Start • Sorting Them Out • New Breed • Motivation • Can • Message Reliability • DeviceNet Connections • Device Net Messages • Stringing Messages Together • Some Real Messages • Object Library • Conformance Testing • DeviceNet Standards • References,Sources,PDF

New Breed

DeviceNet represents a new breed of device networks that offer nifty features, such as hot-plug capability, network-powered devices, peer-to-peer, fiber optics, and fault containment. And if you do it right, your device will be interchangeable with your competitor’s. This may not seem good to you, but your users will like it.

Most device networks, including DeviceNet, are deterministic. In this context, "deterministic" simply means that the network can guarantee a drop-dead maximum delivery time for a critical message. Many peer-to-peer networks can’t claim this because of the possibility of multiple, destructive collisions.

Ethernet using standard hubs, for example, is collision-based and therefore not deterministic. But it can be made so, thereby becoming a contender for real-time control automation.

If you have a fast processor, combined with Java, and perhaps Windows CE, most of the network code is done for you. I’m more of an 8-bit man myself, but with ’386EXs at $16, it’s worth considering.

Most new networks, again including DeviceNet, use a producer-consumer (also called data-centric) model as opposed to the older source-destination model. The data is considered central to the message and is what is identified, rather than the source and destination.

This situation increases the effective bandwidth of the network by permitting one-to-many broadcast messaging and time synchronization. Figure 2 illustrates the difference between the two message models.