PROGRAMMABLE CONTROLLER

PROBLEM TO BE SOLVED: To speedily detect a software run away and speedily take measures against trouble after disconnecting a faulty system immediately. SOLUTION: A sequence engine 22 always judges whether or not trouble occurs on a handshake basis. The sequence engine 22 if detecting trouble inhibits an MPU 1 from accessing an I/O memory 5 by using a bus controller 21. Then the sequence engine 22 judges whether or not write address is needed to stops software PLC itself and ends the process when the write access is not necessary, and further judges whether or not I/O update is needed when the write access is necessary. The sequence engine 22 when judging the I/O update is necessary updates the I/O memory 5. When it is judged that the I/O update is not necessary, on the other hand, the software PLC itself is stopped and the process is finished.