In computing, triple modular redundancy (TMR) is a fault tolerant form of N-modular redundancy, in which three systems perform a process and that result is processed by a voting system to produce a single output. If any one of the three systems fails, the other two systems can correct and mask the fault. If the voter fails then the complete system will fail. However, in a good TMR system the voter is much more reliable than the other TMR components. Alternatively, if there is another stage of TMR logic following the current one (for example, in systems such as the Saturn Launch Vehicle Digital Computer), then three voters are used – one for each copy of the next stage of logic.
The TMR concept can be applied to many forms of redundancy, such as software redundancy in the form of N-version programming.
Some ECC memory uses triple modular redundancy hardware (rather than the more common Hamming code), because triple modular redundancy hardware is faster than Hamming error correction hardware.[1] Space satellite systems often use TMR,[2] [3][4] although satellite RAM usually uses Hamming error correction.[5]
To utilize triple modular redundancy, a ship must have at least three chronometers. At one time, the cost of three sufficiently accurate chronometers was more than the cost of a smaller merchant vessel.[6] Some vessels carried more than three chronometers – for example, the HMS Beagle carried 22 chronometers.[7] Some communication systems use N-modular redundancy as a simple form of forward error correction. For example, 5-modular redundancy communication systems (such as FlexRay) use the majority of 5 samples – if any 2 of the 5 results are erroneous, the other 3 results can correct and mask the fault.
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Triple modular redundancy in popular culture
- The three pre-cogs in Minority Report lead to a conviction, even when one is in the minority.
- To rule out that a single win was "a fluke", some competitions use a two out of three falls match. This isn't true TMR, however, because the three falls are not independent of each other - each competitor knows who has most falls at any point in the competition, which influences their future actions.
- In Arthur C. Clarke's science fiction novel Rendezvous with Rama, the Ramans make heavy use of triple redundancy.
See also
External links
- Article about TMR with reference to TMR usage in avionics and industry
References
- ^ "Using StrongArm SA-1110 in the On-Board Computer of Nanosatellite". Tsinghua Space Center, Tsinghua University, Beijing. http://www.apmcsta.org/File/doc/Conferences/6th%20meeting/Chen%20Zhenyu.doc. Retrieved 2009-02-16.
- ^ "Actel engineers use triple-module redundancy in new rad-hard FPGA". Military & Aerospace Electronics. http://mae.pennnet.com/Articles/Article_Display.cfm?ARTICLE_ID=111934. Retrieved 2009-02-16.
- ^ SEU Hardening of Field Programmable Gate Arrays (FPGAs) For Space Applications and Device Characterization
- ^ FPGAs in Space
- ^ Commercial Microelectronics Technologies for Applications in the Satellite Radiation Environment
- ^ "Re: Longitude as a Romance". Irbs.com, Navigation mailing list. 2001-07-12. http://www.irbs.com/lists/navigation/0107/0066.html. Retrieved 2009-02-16.
- ^ R. Fitzroy. "Volume II: Proceedings of the Second Expedition". p. 18. http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=F10.2&pageseq=41.
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