Withdrawn Standard. Specifications for the subsystem bus of the IEC 821 BUS, referred to as the VSB, are set forth. VSB is a local subsystem extension bus that allows a processor board to access additional memory and I/O over a local bus, removing traffic from the global bus and improving the total throughput of the system. The objectives are: to improve the performance of multiprocessor systems by allowing the design of local subsystems; to specify the electrical characteristics required to design boards that will reliably transfer data over the VSB; to specify the mechanical requirements to be compatible with VSB systems; to specify protocols that precisely define the interaction between the VSB and devices interfaced to it; and to provide terminology and definitions that describe VSB protocols.

The Subsystem Bus of the IEC 821 BUS (IEC 821-1987 [2],1 henceforth referred to as the VSB) is a local subsystem extension bus. It allows a processor board to access additional memory and I/O over a local bus, removing traffic from the global bus and improving the total throughput of the system. The system has been conceived with the following objectives: 1) To improve the performance of multiprocessor systems by allowing the design of local subsystems. 2) To specify the electrical characteristics required to design boards that will reliably transfer data over the VSB. 3) To specify the mechanical requirements to be compatible with VSB systems. 4) To specify protocols that precisely define the interaction between the VSB and devices interfaced to it. 5) To provide terminology and definitions that describe VSB protocols