The Dow Chemical Company

Douglas P Pelletier

Wayne P Dupree

Oscar E Schulze

Johannes C Kanse

Donald J Grinwis

Eloy Baca, Jr

A method of input signal resolution in a process control system where each of the actively redundant process control computers performs a common arbitration process on corresponding input signals. Blocks of corresponding input signals are mutually exchanged by the actively redundant process control computers, and an initial check is made to determine if these input signals are valid. For input signals representing valid analog values, a determination is made as to whether a predefined tolerance has been exceeded between corresponding input signals in the present process control cycle. When the predefined tolerance has been exceeded, then a check will be made to determine the predefined tolerance was also exceeded in the last process control cycle. If the predefined tolerance was not exceeded in the last process control cycle, then the analog input value selected during the last process control cycle will be employed in the present process control cycle. If the predefined tolerance was exceeded in both the present and the last process control cycle, then a predetermined magnitude criteria will be used to select the analog input value for use in making process control decisions. An indication will also be provided in each of the actively redundant process control computers to identify which of the corresponding input signals were selected by that process control computer.

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Midland, MI, United States of America

Douglas P Pelletier