The idea is that during the normal part of the fabric there would be relatively large amount of infrared making it from the source to the analog detector. Then, when a splice occurred, the signal would drop a relatively large amount.
So you would need to use an analog input on the PLC. The code would not be looking for a particular value. Rather, it would look for a radical downward change in the signal. At that time a digital output signal would do whatever it is that the operator is trying to accomplish... unless of course the push button is an input to the PLC. In that case the code detecting the analog signal would simply pass a flag to whatever code the push button applies.
Now, once the splice occurs, you want the code to send only one instance of the "push button" signal to the other code. The concern here is that there might be some "bounce" in the analog signal during the splice.
So, that means that you want to disable the analog detector code for a particular length of time. That time could be calculated (on the fly, or by recipe) relative to the linespeed and then loaded into a timer.