Dell PowerVault 220S (SCSI) - Manual (Page 8)

Introduction: Dell PowerVault 220S and 221S Systems User's Guide
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The only difference between cluster mode and joined-bus mode is the SCSI ID occupied by the enclosure services processor.
When cluster mode is detected, the processor SCSI ID changes from 6 to 15, allowing a second initiator to occupy SCSI ID 6.
As a result, hard-drive slot 13 (normally occupying SCSI ID 15) is disabled, leaving 13 available hard-drive slots in cluster
mode. For more information on SCSI ID assignments and cluster mode operation, see your Installation and Troubleshooting
Guide. See Table 1-4 for a description of the split-bus module modes and functions.
NOTE: To change the SCSI bus mode, you must change the position of the bus configuration switch before turning on
the system. Using the bus configuration switch while the system is on will not affect system operation.
Table 1-4. Split-Bus Module Modes
Mode LED Icon Position of
Bus
Configuration
Switch
Function
Joined-
bus
mode
Top LVD termination on the split-bus module is disabled, electrically joining the two SCSI
buses to form one contiguous bus. In this mode, neither the split-bus nor the cluster
LED indicators on the front of the system (see Figure 1-2 for locations) are
illuminated.
Split-
bus
mode
Center LVD termination on the split-bus module is enabled and the two buses are electrically
isolated, resulting in two seven-drive SCSI buses. The split-bus LED indicator on the
front of the system (see Figure 1-2 for location) is illuminated while the system is in
split-bus mode.
Cluster
mode
Bottom LVD termination is disabled and the buses are electrically joined. The cluster LED on
the front of the system is illuminated while the system is in cluster mode.
The split-bus module has only one LED indicator (see Figure 1-5 ), which is illuminated when the module is receiving power.
Enclosure Management Module (EMM)
The EMM serves two primary functions in your storage system:
SCSI bus expansion — Acts as a buffer for the SCSI bus, electrically dividing the bus into two independent segments
while logically allowing all SCSI bus traffic to pass through it transparently. The buffer improves the quality of the SCSI
signals and allows longer cable length connections.