There exist various situations when it is essential for an operator to be able to trace the control flow step by step in a modern computerized manufacturing system, e.g. during the start-up phase of a new manufacturing cell, when a failure has occurred and the fault should be localized to a certain spot, when a new product is introduced into a running system or when a new machine is installed. An operator should be able to perform these types of manual interaction preferably using a single tool. The idea is that the tool is used by the operator from a centrally located computer, typically the cell computer in a manufacturing cell. The operator should be able to trace the flow of different operations in an operation list in a transparent manner, i.e. without having to consider the actual level of control that he is currently on or on which computer the program is running on.
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