Real choices for real-time networks

f you’ve been successful with your particular brand of fieldbus, you’ll be hesitant to change as long as it continues to meet your performance requirements and is compatible with the drivers, servers, applications, network infrastructure and hardware you already own. But if your process needs improved performance—or real-time functionality, perhaps you should consider a higher-speed version of the fieldbus you already use.

On the other hand, if you’re starting from scratch, you can pick architectures that let you scale up when the need arises. In this case, you have a choice of the old fieldbus stand-bys with their TCP/IP upgrades or a totally Ethernet-based system. Either choice gets you off the old serial cable and onto Ethernet, the ubiquitous networking wiring standard.

A word of warning, though. Just because you choose a standards-based Ethernet at Layers 1 (physical) and 2 (data link) with TCP/IP at Layers 3 and 4 of the ISO’s OSI Table (See OSI Table), you’re not guaranteed that your systems—on the same Ethernet—can talk to each other. Sometimes, however, this lack of communication may be exactly what you want because Ethernet can transmit as many application protocols as you need.

Know Your Requirements
While an industrial network might respond in real time to changes in process variables, a real-time response doesn’t necessarily guarantee a deterministic response within a predefined period every time conditions trigger it. In many cases, having a deterministic cycle will be more important than a real-time response.

For example, real time for a temperature-monitoring circuit may be an update (or cycle time) every 5 to 10 seconds. But real time for motion control may be less than a millisecond—often microseconds. Yet, if a process goes over temperature and causes an excessive pressure condition, you can’t afford to have your network entertain idle chatter from all its nodes—which is often the case with a non-deterministic network such as an office Ethernet. Rather, you’ll need to guarantee that your over-temperature message gets through, and this is where determinism is an important factor in your choice of industrial networks.

Ways to Control Network Traffic
Network nodes need a traffic cop to guarantee that everyone doesn’t talk at once, thus restricting the timely flow of data.

Two methods of controlling traffic are a token-passing arbitration scheme that tells nodes when they can talk, and a master-slave communication system that describes how and to whom they can talk. These OSI Layer 2-based traffic rules or protocols help to guarantee determinism. This is not to say that other arbitration and communication schemes won’t provide determinism and real-time communications but, when it comes to determinism and real-time performance, it’s necessary to consider Layer-2 protocols and overall network speed. In general, faster network speeds bring faster response times. Another rule of thumb: If you need really fast cycle times, keep networks small and use more than one.

According to Bob Bettendorf, former controls engineer and now technical documentation manager for machine builder and automation supplier Mesto Automation, the fundamental reason for separating networks into certain applications is the amount of data that needs to be dealt with in each time interval. For example, he says, Ethernet has quite a bit of overhead in a data packet with a large packet size, so to use it to transmit the 1s and 0s from a proximity switch would result in inefficient use of the network bandwidth. He recommends a network with less overhead and a smaller packet size for that application while the reverse would be true when large chunks of data need to be transmitted.

Pass a Token for Predictable Response
One of the early token-passing networks (IEEE 802.4) was ArcNet, which still is used today. Another token-passing system, MAP (General Motors’ Manufacturing Automation Protocol) died an early death due to lack of product support and $3,000 network nodes. ControlNet is another example of a token-passing network (Concurrent Time Domain Multiple Access, or CTDMA)) that has a total overhead of seven bytes, network update of 2 ms, and frame sizes up to 510 bytes. Profibus-DP/PA is yet another popular token-passing system with update times under 2 ms, depending upon configuration.

Token-passing schemes, which logically place nodes in a circular path, allow each node to receive data from one neighbor and pass it—and any of its own data—along to its other neighbor in a round-robin fashion. The more nodes on the network, the longer the travel time for data to make the trip around the complete circle.