To solve a problem wherein signals transmitted/received between units disappear by extracting a unit having failed, and a problem may occur in a process executed by a unit without having a failure.

A protection signal is connected to signal latch sections 10 of units 6, 9 by an operation protection switch 8, and writing of control/adjustment data into a nonvolatile memory 2 is prohibited during exchange between units 6, 9. A detachable nonvolatile memory 13 retains control/adjustment data of the units during exchange to be set in the changed unit again, and thereby processes the control/adjustment data with the same data as those before the change to operate again, and thereby influence on a line is reduced. Data can be taken over even in changing a control unit because of being detachable.