Functional Overview
PLX Technology, Inc.
4.4.2
Dual-Host Mode
The PEX 8532 can concurrently support two hosts. The PEX 8532 transparent upstream port
is connected to the Active Host, and the NT Port Link Interface is connected to the Backup Host.
After power-up, the Active and Backup Hosts can enumerate their domain at the same time, which
is modified using the serial EEPROM. The serial EEPROM is necessary to enable or resize the
BAR Setup/Limit registers.
Dual-Host mode supports Host-Failover applications, described in Section 4.4.2.1.
Figure 4-8. Dual-Host Model
4.4.2.1
Host-Failover Application
The Host-Failover application is based on the basic Dual-Host configuration. The Active Host
periodically transmits Heartbeat messages, by way of the switch, to the Backup Host to indicate that it
remains active. When the Backup Host fails to receive Heartbeat messages before its Fail Detect Timer
expires, it starts the failover process. The Backup Host halts cross-domain traffic before it starts the
failover. The Backup Host uses the Memory-Mapped access to the PCI Express Capability List and
Capabilities register (offset 68h) to execute the failover. The Backup Host performs the following
procedure to take control:
1. Failover detected.
2. Upstream port demotion.
3. NT Port (self-) promotion as a new upstream port.
Note: Refer to Section 14.2.4, “Host-Failover Application,” for further details.
Dynamic swapping of the upstream and NT Ports is supported on Ports 0 and 8.
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ExpressLane PEX 8532AA/BA/BB/BC 8-Port/32-Lane Versatile PCI Express Switch Data Book
Copyright © 2007 by PLX Technology, Inc. All Rights Reserved – Version 1.6
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