5.7牢靠操控
为使PLC能牢靠作业，在硬件与软件两个方面PLC厂家都采纳了许多办法，对一些特别牢靠请求的PLC，还有相应的特别的办法，如热备、冗余等等。这在介绍PLC的特色时已作了叙说。牢靠办法的意图是添加PLC均匀毛病间隔时刻、MTBF（MeanTimeBetweenFailure）及削减PLC的均匀修正时刻、MTTR（MeanTimeToRepair），以行进PLC的有用度A（Availability）。
A=MTBF/(MTBF+MTTR)
式中A--有用率
MTBF--均匀毛病间隔时刻
MTTR--均匀修正时刻
当然，A值越大越好，它可使PLC体系得到充沛的运用，是为啥要运用PLC的首要方针。而从上式可知，MTBF越大，MTTR越小，则A越大。所以，PLC的牢靠办法都是环绕行进MTBF及MTTR值进行的。

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