s collective Рselective control method,and the latter uses dispatch bined with collective Рselective control method. Р Here the system is to control a 9-storey building,so we choose two Siemens РS7-200 PLCs(CPU226)and its Extensive Modules to control the single elevator Рrespectively.And by using PPI Protocol to realize munication between 2 РPLCs. Р The PPI Protocol adopts master-munication mode,so we defined Рelevator A as the master and elevator B as the slave.munication program,the 2 РPLCs can exchange the massagesuch as the current position, hall-calls or car-calls Рand moving direction.Then by using “minimumwaiting time ”algorithm,thesystem Рrealizes the optimal operation of 2 elevators. Р Figure7 shows the ladder program of the car-calls extreme value calculation of Рelevator A. Р 10
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