IBM RS/6000 - Manual (Page 163)

Mixed-width internal attachment is supported concurrently with single width external
attachment as long as maximum cable length restrictions are not exceeded.
7.2 Cabling the SCSI-2 Fast/Wide Single-Ended PCI Adapter
SCSI Fast/Wide devices support data rates of up to 10 MB/sec for 8-bit or 20
MB/sec for 16-bit transfers. If a configuration includes SCSI-2 Fast/Wide devices,
the maximum cable length supported is three meters (approximately 10 feet). The
maximum length includes the internal cabling of any device that has two SCSI
connectors.
When attaching external devices, a maximum of four independent physical
enclosures is allowed, provided each physical enclosure presents one device load
to the SCSI bus. For example, a 7131-105 cannot be attached in combination with
any other internal or external load, but up to four different boxes, such as the
7204-112, can be attached.
If four external devices are attached and any one device is capable of SCSI-2 Fast
transfers, then the total bus length is limited to three meters. In this case, the
maximum bus length has been reached, and therefore no additional external or
internal device can be attached to the SCSI bus.
A single enclosure containing any amount of multiple SCSI device loads attached
externally to this adapter is supported, but subject to the following restrictions:
Maximum combined internal and external cable length is three meters.
Cable length between devices (loads on the cable) must be 0.1 meters apart at
a minimum.
No mixing of devices widths (8-bit and 16-bit) unless the 68-pin-to-50-pin
interposer (P/N 92F2565) is used.
No internal devices are allowed to be attached to the adapter.
It is recommended that dedicated adapters be used for external SCSI
enclosures that contain more than two devices.
The following tables describe the cables and terminator features, part numbers, and
lengths for the SCSI-2 F/W Single-Ended PCI adapter (FC 2408).
7.2.1 Adapter-to-First Device Cables
This table describes the cables for the SCSI-2 Fast/Wide Single-Ended Adapter for
connection to the first device.
Chapter 7. SCSI Cabling 139