大量的工程实践表明，plc外部输入、输出设备的故障率远远高于plc本身的故障率，而这些设备出现故障后，plc一般不能觉察出来，可能使故障扩大，直至强电保护装置动作后才停机，有时甚至会造成设备和人身事故。停机后，查找故障也要花费很多时间。为了及时发现故障，在没有酿成事故之前使plc自动停机和报警，也为了方便查找故障，提高维修效率，可用plc程序实现故障的自诊断和自处理。

现代的plc拥有大量的软件资源，如fx2n系列plc有几千点辅助继电器、几百点定时器和计数器，有相当大的裕量，可以把这些资源利用起来，用于故障检测。

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