IBM Midrange System DS4000 Series Hardware Manual page 108

When six enclosures are required, the same method is employed again, maintaining drive
channel redundancy and using both controllers, as shown in Figure 3-67.
Figure 3-67 DS5020 drive-side cabling with six EXPs
Drive-side cabling example
As shown in Figure 3-67, the DS5020 is cabled using all two-drive channel pairs, assuming
that there are six expansion enclosures (EXP) evenly spread out across the drive channel
pairs (three enclosures each).
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IBM Midrange System Storage Hardware Guide
Port 2, drive channel 1
Port 1, drive channel 1
DS5020
EXP
EXP
EXP
EXP
EXP
EXP
Port 1, drive channel 2
Port 2, drive channel 2
Drive channel
pair #1
ESM module
Drive channel
pair #2