The Dow Chemical Company

Dow Benelux N.v.

Oscar E Schulze

Dale H Schultz

Leonardus A Van Weele

Ronny P De Bruijn

Roger R Vermeire

Brian G Bell

Marc L Verboven

Wayne P Dupree

Donald J Grinwis

Johannes C Kanse

Eloy Baca, Jr

Douglas P Pelletier

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.

Method of input signal resolution for actively redundant process control

A secure front-end communication system which couples a plurality of actively redundant process control computers to a computer network. The system includes a front end computer which is capable of establishing time limited communication contracts with one or more computer entity on the computer network. Each time limited communication contract is based upon an acceptable response to the transmission of an unpredicable signal from the front end computer, such as an encrypted transformation of a psuedo-random number generated by the front end computer. A security table is used to identify the network entities that are permitted to send write command messages to the process control computers to which the front end computer is connected. The front end computer also includes at least one permissive table which is used to determined whether a write command message from the network entity should be transmitted to the process control computer for which the message was intended.

System and method for determining whether to transmit command to control

Secure communication system for re-establishing time limited

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Midland, MI, United States of America

Oscar E Schulze