By Jim Montague, Executive Editor
Heres another little secretreporters and editors love hate mail because even a negative response proves that someone is reading their articles, which is better than worrying that no one is. In my present incarnation, Ive been covering industrial networks for about 10 years, and Ive grown increasingly disillusioned because it seems like twisted-pair fieldbuses and vanilla Ethernet and its quasi-proprietary flavors arent making substantive gains on the point-to-point hardwiring theyve long been scheduled to replace. So I have to wonder whether anyone is listening or implementing digital networks.
Sure, the protocol organizations, members and suppliers all point to gains made by their particular fieldbuses, and truthfully the several major players have secured millions of nodes. However, all the research Ive seen indicates that two-thirds to three-quarters of all industrial networks remain hardwired. And most observers Ive met acknowledge that fieldbuses should be a lot further along the road to complete victory over hardwiring. So, whats the holdup?
Some fieldbus experts say that, by now, if there are any users who dont know about digital networks at this late date, its because they dont want to know. This bleak analysis is partially true, but I think it also shifts the blame onto the users for many fieldbuses persistent shortcomings. Some users genuinely still dont know about the benefits that fieldbuses can provide, but others dont believe digital networks can help them because its also partially true that many fieldbuses dont function as well on the plant floor as they do in PowerPoint slides. In fact, several system integrators tell me most applications they encounter are hardwired and that fieldbuses cant handle all the nodes and devices they boast that they can support or generate the savings they promise. Fieldbus supporters counter that digital networks deliver most savings over time by improving diagnostics, operations, maintenance and troubleshooting and that integrators and users are still just being stubborn.
Whatever your perspective, implementing a digital network takes some forethought. If youre considering a fieldbus project, there are several basic design and physical description parameters that should be part of any thorough search for the right digital network. These factors include but arent limited to:
Topologyoptions include trunk with drops, linear/daisy chain, star/hub-and-spoke and others. Physical dimensions of application should be sketched out. For example, standalone machines might use daisy chain, a conveyor might use a trunk, and a floor-wide application might need a star.
Speeddepends on whether I/O points need to be scanned every few milliseconds, every few hundred milliseconds or at other intervals.
Data packet size and volumedetermined by how many bits, hundreds of bytes or other packet sizes each I/O device generates, as well as how its data is organized, possibly encapsulated, addressed, transmitted and received.
Network organizationincludes priorities and rules for how components announce and maintain their presence, interact and hopefully avoid data collisions. Methods include master-slave, peer-to-peer, publish-and-subscribe and others.
Protocolsmajor remaining types include AS-i, DeviceNet, Foundation fieldbus, HART, Modbus, Profibus and both corresponding and unrelated Ethernet flavors. Though openness and Ethernet have expanded application types for each, theyre still adopted largely according to historical precedent.
Power distributionmany networks deliver 24 V or loop current; others do not. Recent efforts include distribution of power over Ethernet (PoE).
Cable lengths, connections, terminationshardware availability and limits are usually a function of topology and protocol selected.
Switching and trafficmany new tools are available for analyzing network segment loads and checking to see that communications are running properly.
Though some added pre-planning is needed, users installing digital networks tell me its not as difficult as they expected and the benefits make it more than worthwhile. So, good luck, and let us know how it goes. Theres always someone else whos further back on the learning curve, and they need to know, too. Nasty letters are fine.