INPUT/OUTPUT UNIT FOR PROGRAMMABLE CONTROLLER

PURPOSE:To improve the reliability of control by connecting a gate outputting a specific logical signal to a data bus and making a CPU unit read a specific logical status of an unused address. CONSTITUTION:The CPU unit 12 incorporating a CPU10 executing and operating a user program is provided with an I/O unit 18 through a data bus 14 and an address bus 16. An address signal from the CPU unit 12 specifies and activates an I/O device 22 selectively through a decoder 20 in the I/O unit 18 and the logical status (ON or OFF) of the I/O device 22 makes the CPU unit 12 execute operate and monitor a user's program. In this case, one Tn of output terminals of the decoder 20 is coupled with the data bus 14 through a gate 24 without being connected to the I/O device 22, and when an unused address is specified from the CPU unit 12, the terminal Tn outputs a specific (OFF) logical signal to the data bus 14 through the gate 24.