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Hi There
We here at digboi refinery(Assam-india) have 3 mark-iv installations for our three 8.5 mw gts.

Recently one problem came. All the FSR related tags, logic on s-processor were reported voting mismatch. This phenomena happened suddenly and stayed for 2/3 mins. Then always the s-processor started following the other two processors(R,T),
One day peculiarly all FSR related constants in s-processor went down to the lowest and other logic even like L-4 went down.

I rebooted the s-processor online. But the process persisted. Can anybody help me trouble shooting the problem.

Being out of touch with Mark IV for more than 15 years, I can't memorize the fine details.
If your system is Mark IV (not Mark IV Plus), it will have batterry backed RAM (called BRAM). It could be a BRAM problem. Otherwise replace the processor board.

On the later model with EEPROMs, looks like a problem with the processor board - HMPJ

Another possible area is the communication board HCMC.

You need to use a combination of the Mark IV Speedtronic Elementary, the Diagnostic Alarm Display (the oldest alarm is at the bottom of the list, meaning the beginning of the event will be at the bottom of the list of alarms), and the Logic Forcing Display to see the values of the different signals in the three processors. If L4S is dropping out, then you need to start working backwards from L4T to see what is tripping L4.

Another useful tool is the Auxiliary Display (though most sites have not kept all the display digits working properly). You can look in the Maintenance Manual and/or the Users Manual for instructions on how to use the Aux. Display to see the Process Alarm queue in each processor (remember: two of the three processors must agree that a particular process alarm must exist for it to be annunciated on the <OPM> and logged to the printer; so by using the Aux. Display to see the Process Alarm queue in <S>, you can see which trip condition that only <S> believes has occurred--but it should also be listed in the Voting Mismatch Diag. Alarms).

The information's there; you just need to use the tools to find it.