Patent ID: 7787968

Claim:
A safety PLC capable of rewriting sequence programs, comprising: input means for inputting signals relating to the conductive states of a plurality of input devices; storage means for storing sequence programs each configured by combining arbitrary input devices selected from the inputted input devices in serial or parallel and for storing input device information which classifies each of the input devices as providing either first information indicative of safety ensured or second information indicative of safety not ensured; and arithmetic means for obtaining an arithmetic result based on a sequence program and signals relating to the conductive states of respective input devices; and output means for outputting the arithmetic result of the arithmetic means from an output section to control an output device, wherein the safety PLC judges whether or not the arithmetic result outputted to the output section is of a configuration ensured to be safe, based on stored rules for identifying the arithmetic result as safe or not safe, the stored rules considering the sequence program and the input device information and, when judging that the configuration is not ensured to be safe, the safety PLC sends an output to alerting means and controls an output device connected to the output section toward a safe side.