I/O UNIT OF PROGRAMABLE CONTROLLER

PURPOSE:To provide an I/O unit making an input/output contact part modularized and duplexed, to usually operating one side as a present system and making the other side wait as a stand-by system, and capable of automatically switching from the present system to the stand-by system when an impediment occurs in the present system. CONSTITUTION:Modules constituted of plural input/output contact elements not shown in the figure are loaded on the module attaching/detaching holes 5a, 5b of the I/O unit 1, and one side between loaded two modules becomes the present system module, and the other side becomes the stand-by system. When operation abnormality or a life defect occurs in the present system module, the stand-by system module is switched over automatically to the stand-by system module, and an normality/abnormality display 2 by an LED, etc., for informing a maintenance person that the life defect or the operation abnormality occurs, is displayed.