Patent US 5,185,693

In a backup mode of operation a process control program sends messages, via a local area network, to a process control program running in an active mode for the purpose of obtaining current values of process control data and configuration information. If the active process control program fails to respond to an interrogation by the backup process control program the backup program automatically activates itself to assume the active role. The activation procedure includes a sequence that transmits a control pulse of predetermined width on a line of an associated process control bus to a novel digital data switch. The receipt of a valid control pulse causes the digital data switch to allow access by the backup (now active) data processor to process control I/O unit(s) while simultaneously isolating the active (now backup) data processor from the I/O units. If a response is received the backup processor receives and stores within a memory current process related data such that in the event the backup processor is required to assume the active processor functions the backup processor is fully cognizant of the current state of the process control environment. The switch may also include a watch dog timer for monitoring activity of at least one bus signal line associated with the active data processor for indicating an occurrence of a failure of the active data processor.