IBM RS6000 Service Guide - Page 134
Loops and Data Paths
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Loops and Data Paths All devices that are attached to an SSA adapter card are connected through SSA links. See Figure 3-2 on page 3-60. The SSA links are configured as loops. Data and commands to a particular device pass through all other devices on the link between the adapter and the target device. Data can travel in either direction around a loop. The adapter can, therefore, access the disk drive modules through two data paths. The using system cannot detect which data path is being used. If a disk drive module fails, or is turned off, the loop is broken, and one of the data paths to a particular module is no longer available. The disk drive modules on the loop continue to work, but an error is reported to the using system. Figure 3-3 shows that disk drive module in slot 3 has failed. The disk drive modules in slots 1 and 2 can communicate with the using system only through connector A1 of the SSA adapter. The disk drive modules in slots 4, 5, and 6 can communicate only through connector A2 of the SSA adapter. Figure 3-3. Disk Drive Module in Slot 3 Has Failed 3-62 Service Guide