Programmable controllers are typically connected to industrial equipment, such as assembly lines and machine tools, to sequentially operate the equipment in accordance with a stored program. In programmable controllers such as those disclosed in the above cited patents, for example, the control program is stored in a memory and includes instructions which are read out in rapid sequence and executed to examine the condition of selected sensing devices on the controlled equipment, or to energize or deenergize selected operating devices on such equipment contingent upon the status of one or more of the examined sensing devices.
The processor for these controllers is designed to rapidly execute programmable controller type instructions which in medium to large sized controllers includes not only instructions that manipulate single-bit input and output data, but also arithmetic instructions, file handling instructions, timers and counters, sequencers and other, more complex instructions. Such instructions have become quite standardized in the industry and they may be directly associated with elements of a ladder diagram which is easily understood by control engineers. Program panels such as those disclosed in U.S. Pat. Nos. 3,798,612 and 3,813,649 and in U.S. Pat. No. 4,070,702 have been developed to assist the user in developing and editing ladder diagram type control programs comprised of such programmable controller instructions. To insure that the programmable controller can respond quickly to change in the status of sensing devices on the controlled system, it is imperative that the controller execute the control program repeatedly at a very high rate. The rate at which a programmable controller can execute the instructions in its instruction set, as well as the size of the control program, are the primary factors which determine the rate at which the programmable controller can repeatedly execute, or "scan", the control program.
The sensing devices that are interfaced to the programmable controllers detect various conditions or states of he equipment being controlled and provide input data for use by the ladder program. In response to the sensor data, the program causes the actuators to activate and deactivate various parts of the equipment. The sensors typically are switches, such as limit switches that open or close as a part of the machine or a workpiece passes by the sensor. The status of the switch, open or closed, is represented in the programmable controller as a high or low level voltage which may be digitally processed as a logical one or a zero.
In certain environments electrical signals are hazardous, such as in chemical processes where a spark could cause an explosion. Therefore, fiber optic control systems have been considered for use in such environments. A fiber optic sensing system could send light from a remote source through a fiber optic cable into the hazardous environment. There a shutter type sensor can transmit or block the flow of light into a second fiber optic cable in response to the sensed condition. The second cable is coupled to a I/O module of the programmable controller.
As with other types of sensor systems, the optical link and its various components degrade over a period of time. The optical couplings and the sensors may increasingly attenuate the light transmission with time which, if allowed to continue, can result in a failure of the sensor link. A technique is required to dynamically monitor the fiber optic sensor link and provide an indication of the onset of its degradation so that the faulty components may be replaced before a catastrophic failure occurs.