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Software Synchronization of Multiple Computers

IP.com Disclosure Number: IPCOM000077707D
Original Publication Date: 1972-Sep-01
Included in the Prior Art Database: 2005-Feb-25
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Franklin, EJ: AUTHOR

Abstract

When performing critical process control operations or whenever very high reliability is required, multiple computers performing the same task may be run in synchronism and their outputs compared to detect errors. The majority of the outputs which are the same can be recognized as the correct output. Synchronous operation has been attempted by providing phase-lock circuitry between each computer clock. However, because of signal propagation delay times, the computers will run slightly out-of-phase with each other. Also any given external event will manifest itself to the individual computers at slightly different times, due to the differing signal propagation delays caused by different computer physical locations.

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Software Synchronization of Multiple Computers

When performing critical process control operations or whenever very high reliability is required, multiple computers performing the same task may be run in synchronism and their outputs compared to detect errors. The majority of the outputs which are the same can be recognized as the correct output. Synchronous operation has been attempted by providing phase-lock circuitry between each computer clock. However, because of signal propagation delay times, the computers will run slightly out-of-phase with each other. Also any given external event will manifest itself to the individual computers at slightly different times, due to the differing signal propagation delays caused by different computer physical locations. Therefore, if an external event occurs at the transition of an instruction which recognizes an external event to or from an instruction which prevents recognition of the external event, there is some probability that one computer will recognize the external event, and the other computer will not. Therefore the computers will take different program paths, even through their clocks are in frequency synchronism. These divergent program paths will be detected as hardware errors, because the computers will be providing different outputs.

Synchronization can be accomplished by providing a plurality of discrete synchronizing lines from each computer to all other computers, and a software synchronization routine in each computer. The software synchronization routine places an encoded synchronization signal on the discrete lines every time selected subroutines are entered. Examples of selected subroutines might be, 1) interval timer interrupt occurring for example, every 20 to 100 mi...