Synchronizing slave processors through eavesdrop by one on a write request message directed to another followed by comparison of individual status request replies

Claim 1

In a process control system, having a master controller and at least one pair of slave input/output processors (IOPs) wherein a first IOP of the pair is a primary slave IOP and a second IOP of the pair is a secondary slave IOP, the first and second IOP having a first and second data base, respectively, the first and second IOP each executing the same tasks utilizing a first and second clocking system, respectively, and further wherein the data bases of the first and second IOPs are synchronized, communications by the master controller being made only to the first IOP including communications which modify the first data base, a method for insuring that the first and second data bases remain synchronized, comprising the steps of: a) communicating a write request type message from the master controller to the first IOP to update the first data base, the message being a write request type message which requests the first IOP to store data included in the write request type message into the first data base; b) eavesdropping on the communications of step (a) by the second IOP; c) when the second IOP recognizes that the message of step (a) is to the corresponding primary slave IOP, accepting receipt of the write request type message by the second IOP; d) executing the received write request type message by the first IOP and the second IOP, respectively, the execution of the received write request type message resulting in an update to the respective data bases with the data provided in said write request type message; e) issuing a status request type message by the master controller to the first IOP; f) issuing a status request type message by the master controller to the second IOP; g) responding to the status request type message from the master controller by the first IOP with a response indicating a current status of the first data base; h) responding to the status request type message from the master controller by the second IOP with a response indicating a current status of the second data base; and i) verifying that responses to the requests of steps (g) and (h) are equal, indicating the update of the respective first and second data bases was completed identically, to permit the master controller to be assured that the first and second data bases remain synchronized after the update of the respective data bases.