The IBM S/390 Sysplex Timer

The IBM S/390 Sysplex Timer, a centralized fault-tolerant time reference used in maintaining time-of-day synchronism between multiple closely coupled IBM S/390´s, is presented. The basic Sysplex Timer organization is quad redundant, and its packaging is duplex. A fully duplicated star interconnect topology provides redundant timer transmissions to every S/390 client system using dedicated fiber optic cables. The technology used in the design of the Sysplex Timer and measurements of the performance of the prototype system used in the validation of the design are presented. The design proved to be economic and robust, maintaining tightly synchronized operation of the redundant components for all test cases. Cable latency compensation appeared to be particularly effective. Worst case operation of the prototype with less than 300 ns of timer skew was verified.<>