PLC HAVING POWER FAILURE COMPENSATION FUNCTION AND POWER FAILURE COMPENSATION METHOD

A PLC having a power failure compensation function and a power failure compensation method which are capable of implementing the continuity of a program operation when a power failure occurs. The PLC includes a first storing unit for storing a program execution data and a user's program executed by the CPU, a power failure detector for checking a state of an alternating current power and outputting a power failure generation signal, a program counter for counting each step of a program executed by the CPU, a second storing unit for storing a count value of the program counter which is the program step value that the CPU finally executes before the power failure, a third storing unit for storing an execution result value of a program obtained in the program step that the CPU finally executes before the power failure, a battery for supplying a direct current power to the second and third storing unit, respectively, and a CPU for reading the step value of the program executed before the power failure from the program counter and storing the thusly read value into the second storing unit, storing the execution result value of the program finally executed before the power failure into the third storing unit, reading the user's program stored in the first storing unit and the data stored in the second and third storing unit, and executing the user's program from the next step of the finally executed program step.