The selection of redundant functions should be considered from two aspects:
(1)Redundancy of the control unit
Important process unit:CPU(including memory)and power supply should be 1:1 redundant;when necessary,a hot-standby redundant system composed of PLC hardware and hot-standby software,2-fold or 3-fold redundant fault tolerance can also be used system,etc.
(2)Redundancy of I/O interface unit
The multi-point I/O card of the control loop should be redundantly configured;the multi-point I/O card of the important detection point can be redundantly configured;according to the needs of the important I/O signals,2-fold or 3-fold I/O can be selected/O interface unit.
对于冗余功能的选择应从两方面考虑：
（1）控制单元的冗余
重要的过程单元：CPU(包括存储器)及电源均应1:1冗余；在需要时也可选用PLC硬件与热备软件构成的热备冗余系统、2重化或3重化冗余容错系统等。
（2）I/O接口单元的冗余
控制回路的多点I/O卡应冗余配置；重要检测点的多点I/O卡可冗余配置；根据需要对重要的I/O信号，可选用2重化或3重化的I/O接口单元。
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