Image Number 6 for United States Patent #4428044.
A synchronization scheme is used in a peripheral unit controller which consists of two duplicated units whose outputs are matched. In addition, each duplicated unit contains a pair of microprocessors whose outputs are also matched. The synchronization scheme allows each microprocessor to run its own diagnostics independently, and to synchronize itself with the other microprocessor of the pair. After the synchronization occurs in both microprocessor pairs, the duplicated units are synchronized. Synchronization is achieved by using a real-time clock and the interrupt structure of each microprocessor.