| 摘要 |
A PLC(Programmable Logic Controller) is provided to enable a user to cope with a unit exchange error by informing the user erroneous setting of a unit using a display lamp or a programmable display. A CPU(Central Processing Unit) unit is connected to plural functional units through an I/O bus(11). Each of the functional units is detachably connected to the I/O bus through connectors(13a~13d). A state memory includes memory regions for the connectors and stores first and second states according to detachment of the functional unit from one of the connectors. When an abnormal response is detected, the CPU accesses the functional unit and a first controller refers to the state memory. The first controller performs a unit detachment process when the stored information is the first state, and processes a communication error process when the stored information is the second state. When the stored information is changed from the first to the second state, a second controller performs a unit attachment process on a specified connector, while a PLC is running.
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