Re: Fibre optic data
If you have an oscilloscope start with the 87C51 TX pin, then go to the input of the fibre optic modem, and so on.
By doing this you will find out where the commands are stopped, or not generated at all - if the mcu is faulty..
It looks for me like a thermal fault, so you can arm yourself with a heatgun and a freazing spray.
Heat up crucial components in your circuit and simulate this fault .. again, start with the 87C51 ..
Regards,
IanP