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Chapter 29 ½ The Tandem 16 471

needed when applications are to be geographically changed or expanded.

This paper describes the Tandem 16 architecture at the lowest level (the hardware). Section 1 deals with the overall system organization and packaging. Section 2 explains the processor module organization and its attachment to the interprocessor communications system. Section 3 discusses the I/O system organization. Section 4 discusses power, packaging, and on-line maintenance aspects that are not covered elsewhere in the paper.

1. System Organization

The Tandem 16 NonStop system is organized around three basic elements: the processor module, dual-ported I/O controllers, and the DC power distribution system (Figs. 2 and 3). The processors are interconnected by a dual-interprocessor bus system: the Dynabus; the I/O controllers are each connected with two independent I/O channels, one to each port; and the power distribution system is integrated with the modular packaging of the system.

The system design goal is two-fold: (1) to continue operation of the system through any single failure, and (2) to be able to repair

that failure without affecting the rest of the system. The on-line maintenance aspects were a key factor in the design of the physical packaging and the power-distribution of the system.

System Packaging

The cabinet (Fig. 4) is divided into 4 sections: the upper card cage, the lower card cage, cooling, and power supplies. The upper card cage contains up to 4 processors, each with up to 512K bytes of independent main memory. The lower card cage contains up to 32 I/O Controller printed circuit (PC) cards, where each controller consists of one to three PC cards. The cooling section consists of 4 fans and a plenum chamber that forces laminar air flow through the card cages. The power supply section contains up to 4 power supply modules. Multiple cabinets may be bolted together, and the system has the capability to accommodate a maximum of 16 processors.

Each processor module, consisting of CPU, memory, Dynabus control and I/O channel, is powered by an associated power

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