Method of maintaining synchronization of a free-running secondary processor

In a process control system, having a master controller and at least one pair of slave input/output processors (IOP's) 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 maintaining synchronization between said first and second IOP comprising the steps of:

• a) transmitting a message addressed to the first IOP by the master controller;
• b) receiving the message by the first IOP for subsequent execution;
• c) acknowledging receipt of the message by a transmission of an acknowledge message back to the master controller, the acknowledge message including a message number;
• d) eavesdropping, by the second IOP, on transmissions from the master controller to the first IOP, and on transmissions from the first IOP to the master controller;
• e) upon recognition that the message is addressed to the corresponding first IOP and that the message is of a predetermined type, receiving the message by the second IOP for subsequent execution;
• and verifying the message number in the acknowledge message by the second IOP, the acknowledge message for the first IOP to the master controller having been received by the second IOP as a result of the eavesdropping step, the message number being determinable by the second IOP, such that when both the first and second IOP complete execution of the received message which results in an update of the respective data bases, both data bases continue to contain the same information thereby maintaining synchronization.