IBM RS/6000 - Manual (Page 162)

Devices which have two SCSI connectors have internal cabling which must be
included when calculating the total cable length. When attaching these devices to
a SCSI chain, connect one cable to one connector and the other cable to the other
connector.
7.1.2 SCSI Terminators
There must be exactly two terminators on the SCSI bus, and they must be located
at each end of the bus.
For SCSI adapters with external devices only, make sure the appropriate SCSI
terminator is connected to the last device on the bus.
For SCSI adapters with internal devices only, make sure that the appropriate SCSI
terminator is connected to the end of the internal SCSI cable.
For SCSI adapter with both internal and external devices, make sure the
appropriate SCSI terminator is connected to the last device on the external bus and
that the end of the internal SCSI cable has been properly terminated.
7.1.3 SCSI Device Addresses
The SCSI-2 Fast/Wide adapters support a maximum of 15 SCSI addresses,
including devices and adapters.
All devices on the SCSI bus must have a unique SCSI ID.
The SCSI bus address determines priority on the bus. Address priority from
highest to lowest is as follows:
– 7, 6, 5, 4, 3, 2, 1, 0 and 15, 14, 13, 12, 11, 10, 9, 8, with addresses 15
through 8 being used in 16-bit configurations only.
– Generally, the highest priority is assigned to the adapter.
– Disk drives should be assigned IDs in the range of 0 to 3, CD-ROMs in the
range of 3 to 4, and tape drives in the range of 4 to 6.
For any single adapter, in a 8-bit configuration, a maximum of seven devices
are permitted, provided that the supported configuration-specific bus lengths
are not exceeded.
For any single adapter, in a 16-bit configuration, a maximum of 15 devices are
permitted, provided that the supported configuration-specific bus lengths are not
exceeded.
Refer to 2.4.3, “RS/6000 Model E20 Limitations” on page 31 and 2.5.3, “RS/6000
Model F30 Limitations” on page 38 on Chapter 2, “PCI-Based RS/6000 Server
Hardware” on page 11 to know the maximum number of devices that can be
connected to PCI SCSI-2 adapters supported by the PCI-based RS/6000 servers.
7.1.4 SCSI Bus Width
Operation of both 8-bit and 16-bit devices on the same external SCSI bus is not
supported due to the termination and cabling restrictions.
Operation of both 8-bit and 16-bit devices on the same internal bus is supported
concurrently as long as 16-bit internal cable and 68-pin to 50-pin interposers are
used.
138 Introduction to PCI-Based RS/6000 Servers