HVAC Control in the New Millennium.pdf - HVAC.Amickracing

Advanced HVAC Control, Information Technology and Open SystemsFigure 4-9. Shared-file cluster configuration.Systems that mask failures use parallel components. Each componentis capable of doing the job. If one fails, the other continues.The backup systems can be connected by some type of network connection,such as high-speed Ethernet. The data stored on the systemsare mirrored over this path which also carries heartbeat messagesfrom each system on their mutual status. This technique is used inthe Vinca cluster and in the Octopus backup system.Active redundant, fault-tolerant systems use at least two processorsand custom hardware in a fail-stop configuration (Figure 4-10).Two processors are used in microcycle lockstep with hardware comparisonlogic to detect an error in the output of the two systems.If the output disagrees, the system is stopped. Fail-stop systemsare used by Stratus and Tandem. A special OS manages faults, errors,and system synchronization. The applications that run on these systemsrequire modifications in order to use the fault-tolerant features.Assured availability servers identify errors or failures before anydata become corrupt. These systems isolate errors or failures so they©2001 by The Fairmont Press, Inc. All rights reserved.