ls.The former uses collective selective control method,and the latter uses dispatch bined with collective selective control method. Here the system is to control a 9-storey building,so we choose two Siemens S7-200 PLCs(CPU226)and its Extensive Modules to control the single elevator respectively.And by using PPI Protocol to realize munication between 2 PLCs. The PPI Protocol adopts master-munication mode,so we defined elevator A as the master and elevator B as the slave.munication program,the 2 PLCs can exchange the massage such as the current position, hall-calls or car-calls and moving direction.Then by using “ minimum waiting time ” algorithm,the system realizes the optimal operation of2 elevators. Figure7 shows the ladder program of the car-calls extreme value calculation of elevator A.
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