Patent ID: 7990880

Claim:
A detector for detecting a time lag between a first timer of a first device and a second timer of a second device, the first device and the second device being connected to each other through a communication line, the detector comprising: a transmitting and receiving section for transmitting data to which a first time measured by the first timer of the first device is attached to the second device through the communication line, and receiving the data at the second device; a communication abnormality detecting section for detecting a transmission delay of the communication line between the first device and the second device when a difference between the first time attached to the data and a second time measured by the second timer of the second device at the time of receiving the data exceeds a first threshold value; and a time lag detecting section for detecting the time lag between the first timer of the first device and the second timer of the second device when the difference between the first time and the second time is in a range of a second threshold value to the first threshold value, the second threshold value being smaller than the first threshold value, wherein the time lag detecting section notifies a pre-alarm when the time lag is detected, and wherein the communication abnormality detecting section initiates a shut down of a plant when the transmission delay is detected.