Forum,
I've interfaced two Fairchild H21A1 phototransistor optical interrupter switches to a T89C51RD2 chip on P3.5 and P3.7 as follows:
5V through 270 ohm resistor to pin 1 (anode)
ground to pin 2 (cathode)
ground to pin 3 (collector)
port pin of T89 to pin 4 (emitter)

PROBLEM: This worked for about 4 months, but yesterday one of the port pins started not detecting a change (always a high signal or led blocked condition). I changed the emitter/detector pair and this has cured the problem for now. The LED is still working so the detector must have failed.

QUESTION: Why did the detector fail, if it is rated for 20 mA collector current? Design fault or bad component?

I measure .018 mA through the emitter to the port pin when the led is blocked. .001 mA when it is not blocked.

These two switches indicate up and down positions of a mechanical movement. It spends more time in the up position and this is the switch that failed.
Any thoughts?
Thanks for your help.
Jon