Careful and clever analysis of fieldbus segments can yield fault tolerance where it's needed most without adding hardware costs over less effective strategies.
　　It's ironic: Network cables in a safe and well-managed control room environment are almost always made redundantwww.cechina.cn, whereas field cables exposed to the harsh and sometimes corrosive environment of a modern industrial plant have to fend for themselves. Of course, if those field cables carry simple point-to-point communications such as 4-

20 mAwww.cechina.cn, then redundancy isn't really a concern in generalCONTROL ENGINEERING China版权所有, and specific devices can be duplicated as required. However, now that the lowest fieldbus physical layer carries data from up to 32 deviceswww.cechina.cn, the vulnerability of that cable can constitute a reliability issue, particularly if those devices are safety-related or process-critical. Conventional Foundation Fieldbus (FF) segment design does not lend itself to any version of fault- 

　　Duplex wiring is typically the "conventional" approach but does not offer nearly the protection of fault-tolerant wiring. Data on cable failures in industrial environments used for MTBF calculations comes from "Reliability, Maintainability and Risk" 7th Edition, Dr. David J. Smith
　　tolerance except through complete and wholesale duplication. In a fieldbus context, that duplication brings with it special software requirements to implement one-out-of-two and two-out-of-three voting schemes and special measures required for safe maintenance, device replacement, etc.
　　In late 2007www.cechina.cn, MooreHawke Fieldbus released a new fault-tolerant segment design that permits a far higher segment MTBF (mean time between failure) than conventional designs without any special software in the DCS and for only the additional cost of an extra trunk cable. Working with a major DCS provider and a large oil & gas end user, this package was installed on a set of platforms in the South China Sea simply to mitigate the huge financial risk associated with loss of co