A control system comprising: a communicator configured to communicate, via a communication line, with an external device according to a predetermined communication protocol, wherein a delay is generated due to the communication cycle time of the communication line compared to a signal line; a signal receiver configured to receive, directly via the signal line, an output signal that is output from the external device; and a determiner configured to determine whether abnormality is generated in the signal line connecting the signal receiver to the external device based on data received with the communicator and an output signal value received with the signal receiver. characterized by: a detector configured to detect a change of output data corresponding to the output signal value, wherein the output signal value is a binarized digital signal value, the data received with the communicator includes the output data corresponding to the output signal value, the determiner is configured to check whether the output signal value changes between a detected second time and a first time before a predetermined time with respect to the second time, at which second time the detector detects that the output data changes, wherein the predetermined time with respect to the second time is at least double the communication cycle time of the predetermined protocol, and the determiner is configured to determine that the signal line is abnormal when checking that the output signal value changes, and determines that the signal line is not abnormal when checking that the output signal value does not change. a communicator configured to communicate, via a communication line, with an external device according to a predetermined communication protocol, wherein a delay is generated due to the communication cycle time of the communication line compared to a signal line; a signal receiver configured to receive, directly via the signal line, an output signal that is output from the external device; and a determiner configured to determine whether abnormality is generated in the signal line connecting the signal receiver to the external device based on data received with the communicator and an output signal value received with the signal receiver. characterized by: a detector configured to detect a change of output data corresponding to the output signal value, wherein the output signal value is a binarized digital signal value, the data received with the communicator includes the output data corresponding to the output signal value, the determiner is configured to check whether the output signal value changes between a detected second time and a first time before a predetermined time with respect to the second time, at which second time the detector detects that the output data changes, wherein the predetermined time with respect to the second time is at least double the communication cycle time of the predetermined protocol, and the determiner is configured to determine that the signal line is abnormal when checking that the output signal value changes, and determines that the signal line is not abnormal when checking that the output signal value does not change. a detector configured to detect a change of output data corresponding to the output signal value, wherein the output signal value is a binarized digital signal value, the data received with the communicator includes the output data corresponding to the output signal value, the determiner is configured to check whether the output signal value changes between a detected second time and a first time before a predetermined time with respect to the second time, at which second time the detector detects that the output data changes, wherein the predetermined time with respect to the second time is at least double the communication cycle time of the predetermined protocol, and the determiner is configured to determine that the signal line is abnormal when checking that the output signal value changes, and determines that the signal line is not abnormal when checking that the output signal value does not change.