Coordination of time-of-day clocks among multiple systems

The IBM Enterprise Systems Architecture/390™ External-Time-Reference (ETR) architecture facilitates the synchronization of time-of-day (TOD) clocks to ensure consistent time-stamp data in an installation with multiple systems. The ETR architecture also provides a means by which the TOD clocks can be set automatically, without human intervention, to an accurate standard time source. This paper reviews the design considerations involved in providing these functions—along with “clock integrity” and continuous operation—as a consistent extension of the System/370™ TOD-clock architecture. The paper also provides a functional description of the IBM 9037 Sysplex Timer™, which is an implementation of the sending unit of the ETR network.