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Adequate system availability is provided by suitable redundancy of components. The general form of redundancy chosen is a composite form in which the reserve component is operating on its own behalf while ready to take over the tasks of a failed component. In particular, the point-of-sale terminal can pick up the operation of another terminal if failure occurs in midtransaction, and the supermarket controller can assume the load of a failed controller in another store. The retail point-of-sale terminals can operate off-line in the event of controller or loop failure. Each supermarket is served by two loops so that half the number of termipals in the store will be unaffected by a loop failure.
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