IBM System Storage Subsystem DS5020 - Manual (Page 90)

One DS5020 and two storage expansion enclosures
If you are cabling one DS5020 storage subsystem and two storage expansion
enclosures, connect one storage expansion enclosure per controller drive port as
shown in Figure 38. Figure 39 on page 61 shows that you can also connect two
expansion enclosures together behind a single controller drive port. Although this is
technically correct, use the topology as shown in Figure 38.
As Figure 38 shows, when you connect additional storage expansion enclosures,
you can connect them by using the additional port of the drive channel.
When you connect additional storage expansion enclosures, you can connect them
by continuing the connection from the existing redundant drive channel pair.
DS5020
Port 2, drive channel 1
Port 1, drive channel 1
Port 1, drive channel 2
Port 2, drive channel 2
1A 1B
1A 1B
1A 1B
1A 1B
Figure 38. Cabling for one DS5020 and two storage expansion enclosures
60 IBM System Storage DS5020 Storage Subsystem: Installation, User’s, and Maintenance Guide