Abstract: 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.

Abstract: 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.

Abstract: 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.